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Thomas Reid Inquiry And Essays Summary Of The Scarlet

1. Philosophical Method

1.1 Common Sense and First Principles

Reid often articulates his theoretical positions in terms defending common sense and the “opinions of the vulgar”. Indeed, he is often described as a “common sense philosopher”. This reputation owes less to the philosophical uses Reid makes of common sense than to fellow Scotsman James Beattie, who popularized common sense in his very widely read An Essay on The Nature and Immutability of Truth In Opposition to Sophistry and Scepticism (1770). Even today it is easy to overestimate and to misunderstand the role of common sense in Thomas Reid's philosophical system.

The most important use of the term “common sense” in Reid occurs in the context of his epistemology and his philosophical method. Here it refers to a select set of intuitive judgments. Reid calls these variously “first principles, principles of common sense, common notions, [or] self-evident truths” (Essays on the Intellectual Powers, abbreviated ‘EIP’, 6.4, 452). Common sense first principles are identifiable because they typically possess a suite of additional traits (traits, note, not necessary conditions) as follows. Denial of a common sense principle is not only false but absurd (EIP 6.4, 462). A common sense judgment is “necessary to all men for their being and preservation, and therefore it is unconditionally given to all men by the Author of Nature” (EIP 4.6, 412). Common sense intuitive judgments are “no sooner understood than they are believed. The judgment follows the apprehension of them necessarily, and both are equally the work of nature, and the result of our original powers” (EIP 6.4, 452).

Common sense principles possess “the consent of ages and nations, of the learned and unlearned, [which] ought to have great authority with regard to first principles, where every man is a competent judge” (EIP 6.4, 464). Common sense principles are common sense because, but not only because, they are common to humanity. Reid's account of common sense does not itself approach the status of a philosophical or psychological theory. Though his commitment to common sense forms a key component in his philosophical method, a deeper methodological principle clearly underlies his appeal to common sense. Reid attempts to build his fallibilist, foundationalist account of empirical knowledge while working under an empiricist constraint that prohibits speculative use of a priori reason. As a result, Reid's reference to beliefs held in common across time and place, across culture and religion, functions as an empirically justified generalization about species-typical features of human cognition.

Though this marks a point commonly neglected in the interpretation of Reid, one can often find, in his discussion of common sense, emphasis on empirical generalizations from observable data about what people believe and how they behave. For example, he writes, “The universality of these opinions, and of many such that might be named, is sufficiently evident, from the whole tenor of human conduct, as far as our acquaintance reaches, and from the history of all ages and nations of which we have any records” (EIP 6.4, 466). Laying stress upon his claim about the universality of certain beliefs across our species has opened Reid up to tiresome criticism that began in his lifetime and continues to the present of the following sort: Reid's appeals to common sense are little more than affirmations of majority opinion or appeals to the masses, but these are fallacious, so his inferences from such principles are defeated. Reid corrects this misunderstanding by emphasizing the fact that his first principles are psychological generalizations about belief formation applicable to most of our species (EIP 6.4, 464–5).

Reid often appeals to the structure of languages as evidence for generalizations about human cognition, belief, and descriptive metaphysics. Language, being something so widely shared, offers an abundance of data for observation. Reid finds many commonalities across languages. (The connection between ordinary language and common sense that Reid espouses was of great influence on such later philosophers as G. E. Moore and J. L. Austin.) Reid does not believe, however, that every feature of ordinary language is indicative of some important tenet of common sense (EIP 1.1, 26–27). Reid often suggests that the relevant features are those that can be found in “the structure of all languages”, suggesting that the linguistic features of relevance are features of syntactic structure shared among languages. Reid says there is some important difference between the active and the passive, since “all languages” have a passive and active voice. All languages distinguish between qualities of things and the things themselves (EIP 6.4, 466). This suggests that certain universal features of the syntactic structure of languages inform us of a common sense cognitive commitment, even if it is implicit.

Most of Reid's first principles of contingent truth take the form of Principle 5, which reads “That those things do really exist which we distinctly perceive by our senses, and are what we perceive them to be” (EIP 6.5, 476). Such common sense first principles are intended to be more than merely generalizations about how humans across cultures form beliefs. Reid intends these general principles to provide evidence for particular beliefs. Thus Principle 5 issues in the self-evident belief, for example, that I perceive a computer before me (Van Cleve 1999). As a result, Reid's philosophical method accords with common sense insofar as everyday perceptual beliefs are evident or self-evident.

An implication of Reid's application of his common sense method to first principles is that Reid is not concerned to answer questions of justification that appear pressing to contemporary epistemologists. He is not, for instance, interested in providing a refutation of skepticism about the external world by appeal to first principles. Reid believes he can refute skeptical hypotheses--such as Descartes's hypothesis of an evil demon who makes us believe that the world is the way we take it to be when it is really vastly different--simply by showing that such a hypothesis is no more likely to be true than the common-sensical belief that the world is much the way we perceive it to be. Since the belief in the external world is a dictate of common sense, it is, Reid thinks, as justified as it needs to be when it is shown to be on the same footing as any alternative. Justification, therefore, does not necessarily require providing positive reasons in favor of common-sensical beliefs; common sense beliefs could be adequately justified simply by undermining the force of the reasons in favor of alternatives to common sense. In fact, as we move through this entry, we will witness Reid's repeated deployment of this strategy in the form of burden-of-proof arguments against his major foil, the Way of Ideas. Common sense constrains, rather than dictates, acceptable philosophical positions.

A number of additional problems remain in accounting for Reid's appeal to common sense and in his treatment of first principles. For example, a number of comments Reid makes indicate that he appears to have a psychological conception of evidence whereby what is evident forces assent. He writes, “[The different kinds of evidence] seem to me to agree only in this, that they are fitted by nature to produce belief in the human mind” (EIP 2.20, 292). Just what he means, then, by terms “evident” and “self-evident” (terms he greatly prefers to the more contemporary “justification” and “knowledge”) is an issue meriting further research.

1.2 Newtonianism and Empiricism

The relations between one's first principles, the perceived aims—and limits—of natural philosophy, and one's religious background came together in eighteenth-century Great Britain to issue in a number of different philosophical stances. Typically these stances are framed as various commitments to Newton and Newtonianism. Hume and MacLaurin believe the mind's operations are to be studied with broadly observational Newtonian methods, though this leads them to forms of local skepticism. Priestley and Hartley apply Newtonianism not only to the operations of the mind but to the mind's substance via materialist commitments. Reid's teacher George Turnbull adopted Newtonianism and was led to Berkeleyan idealism by many of Berkeley's own common-sense commitments. Wanting both the world and knowledge of it in his philosophical system, Reid was at pains to articulate his account of both common sense and Newtonianism. Unlike most British Empiricists, Reid read, understood, and taught Newton's writings. To understand Reid's philosophical method, not to mention his philosophy of science, one must understand core features of Reid's Newtonianism (Callergård, 2010) and, perhaps, how Reid altered Newton's own method (Ducheyne 2006).

First, Reid is committed to the positive role of mathematics in stating and testing theories. Only in that field do we “find no sects, no contrary systems, and hardly any disputes” (EIP 6.4, 457). Newton's greatness lies in part due to the fact that he places physics upon firm mathematical ground, in sharp contrast to Cartesian physics, its leading competitor of the time. Second, Reid says that issues about causation are not issues physics should attempt to resolve. This counterintuitive commitment is explained by the fact that Reid believes causes, when that term is used properly, are efficient causes (The Correspondence of Thomas Reid, abbreviated ‘Correspondence’ below, 2006, 158). Not only this, efficient causes are only ever agents (EAP 1.5, 30–32; for more about this, see 4. Causation and Free Will, below). Reid attributes this position to Newton. Reid mentions on many occasions, and with a certain pride, that Newtonian science does not permit knowledge of causes of phenomena, for example, the motion of the compass needle or the attraction of two bodies. Here and elsewhere Reid frequently speaks with the scientists and uses ‘cause’ in a colloquial way to refer to physical causes; in doing so, he explicitly follows David Hume's lead. More often, however, Reid urges readers to think of scientific explanation in terms of laws, as Newton had done. Laws are true general propositions used to explain appearances (Thomas Reid on the Animate Creation 1996, 187). Physics does not aim to find efficient causes. By dispensing with causes and amplifying the explanatory value and empirical justification of statements of laws, Reid's account is regarded as a forerunner to a deductive-nomological model of explanation.

Third, related, when one event produces another event, e.g. fertilizer enables better plant growth, Reid strongly resists describing that interaction as necessary (The Correspondence of Thomas Reid 2006, 234, 243). One event may be constantly conjoined to another event, but it is a mistake to believe that this forms any necessity. Fourth, unlike nearly all other Early Modern philosophers traditionally taught in the canon, Reid was an avowed experimentalist, made so by borrowing Newton's methods in Opticks, and conducted experiments to provide evidence for his claims about the nature of the mind, perception and agency. Reid was active in his community, bringing his penchant for knowledge through experimentation to meetings in Aberdeenshire in which experimental techniques in farming were debated. Fifth, Reid understands Newtonian physics to offer partial confirmation to some beliefs about God and God's relation to the world. Newtonian natural science's role in this connection is to provide evidence for belief that our solar system is orderly and well-governed. (See 8. Philosophy of Religion below).

Stating central features of Reid's commitment to Newtonianism goes a long way to understanding Reid's empiricism and science since Reid attributes most of his own views about these matters to Newton. Despite the fact that with a few notable exceptions Reid scholars have neglected issues in his philosophy of science, a few key controversies have emerged and merit brief mention. First, Reid was embroiled in clashes with other thinkers and correspondents about the scope of Newton's Regulae Philosophandi. Interpretations differed considerably, as did the translations and restatements of the rules themselves. Given that after Bacon's work, Newton's Regulae formed the most important statement on method in natural philosophy to be found in the Early Modern period, Reid was quick to defend his interpretation of these rules against alternate uses by Priestley and others.

Reid translates Newton's first rule from Latin as “No more causes, nor any other causes of natural effects ought to be admitted, but such as are both true, and are sufficient for explaining their appearances” and adds in his own voice, “This is a golden rule; it is the true and proper test, by which what is sound and solid in philosophy may be distinguished from what is hollow and vain” (EIP 1.3, 51). Reid expands the intended scope of this rule saying that it is a “fundamental principle in our enquiries into the structure of the mind, and its operations” (EIP 1.3, 51). Occurring in the introductory portion of his Essays on Intellectual Powers, he would go on to use and reuse it countless times in what followed. He also describes Newton's first rule as “a dictate of common sense” (EIP 2.6, 102). Proponents of the Way of Ideas fail to abide by Newton's first rule when they endorse the existence of ideas because the existence of ideas is an hypothesis and lacks evidence.

Another area of controversy in Reid's empiricism involves the grounding of his belief to the effect that ether does not exist. Ether appears to represent just the sort of posit (originally by Descartes in Principles of Philosophy) that Newtonians often enjoyed sweeping into the trash. Lacking any observational evidence for ether, Descartes posits ether as a medium through which forces can act on bodies that are not in direct contact. But does Reid reject ether theory on the grounds that it is unobservable and therefore does not belong in a Newtonian science, or does he reject ether theory because scientists as yet lack justificatory observations on its behalf? Reid's apparent hostility to “hypotheses” in philosophy and “efficient causes” in natural philosophy gives rise to the belief that Reid does not believe that forces should be allowed into philosophy. The answer to this question can resolve that issue and assist in locating Reid's philosophy of science on the positivist/realist spectrum.

The first rule does not entail that unobservables like ether, subtile fluids, or forces do not or cannot have explanatory force, let alone that unobservables do not exist. Reid is explicit that questions about the existence of ether were open questions that were not settled by a priori (EIP 2.3, 82). Physical forces and causes do exist and may yield their secrets to science in the form of purely physical forces of attraction (An Inquiry into the Human Mind on the Principles of Common Sense, abbreviated to ‘IHM’, 7, 211). This has two corollaries of considerable importance for understanding Reid's methods. First, the fact that Reid admits that these forces of attraction—unobservable, immaterial forces—may be physical implies that he is comfortable with a physical explanation of gravitation that does not entail the existence of God. Second, physical forces will not yield their secrets to an impoverished Cartesian science. This criticism is made on the grounds that Cartesians were attempting to understand gravitational attraction in terms of only extension, figure and motion—for Reid, a catastrophic mistake. In fact, Reid's criticism of the Cartesians has provided a clue for an account of Reidian forces as irreducible properties of matter (Callergård 2005, 2010).

2. Conception

Reid's theory of conception is at the heart of his philosophical system and his faculty psychology. The term ‘faculty psychology’ refers, at least for Reid, to both the distinction of certain belief-forming systems from others, and the explanatory utility purchased with such a set of distinctions. While faculty psychology appears to be an obvious way to scientifically study the senses, Reid's study of intellectual powers—memory, judgment, abstraction—under the umbrella of faculty psychology was not obvious. Reid would face pointed criticism for multiplying faculties, but this consequence was more than outweighed by the explanatory utility of separating mental powers—their inputs, their operations, and their outputs—from one another.

The most ubiquitous mental power is that of conception. Our suite of intellectual faculties supports a wide variety of mental events. Acts of conceiving are embedded in most of them. Whether judging that there is a tree before me, imagining there is a tree before me, or reasoning to a generalization that all trees have roots, these mental events employ the faculty of conception. It is in virtue of conception that the first of these mental states is about the proposition there is a tree before me; that the second mental state takes as its object a non-existent tree; and that the third uses a general conception of ‘tree’. The Way of Ideas teaches, wrongly, that acts of perception temporally begin with an act of simple apprehension “and that after we have got simple apprehensions, by comparing them together, we perceive agreements or disagreements between them,” but, Reid continues, “this appears to me to be all fiction, without any foundation in nature” (IHM 2.4, 29). Instead, simple apprehension—the basic form of conception—is bundled within typical acts of perception. The model of conception received from the Way of Ideas marks its first long step into two types of skepticism.

Note that the set of commitments that Reid refers to as “The Way of Ideas” and the “Ideal Theory” is drawn form the work of Locke, Berkeley, Hume and many others going back to Plato and Aristotle. Its principle commitment is to mental representations, called ‘ideas’ or ‘images’, that are believed to mediate all our experience of the world—from conception to memory and perception. Reid regards the Way of Ideas as founded on a flawed, anti-Newtonian methodology, is unscientific, and he argues that it has highly undesirable consequences.

2.1 Conceiving

Conception lies at the heart of the operations of the intellectual faculties because conception provides the intentional content—the ‘aboutness’—to mental states. Reid distinguishes between several functions of conception. A rudimentary form of conceiving, which Reid often calls ‘simple apprehension,’ refers to “the bare conception of a thing without any judgment or belief about it” (EIP 4.1, 295). This “enters as an ingredient in every operation of the mind” (EIP 4.1, 296). In order to believe, or remember or perceive, I must perform an act of simple apprehension in order to get something in mind.

We explain Reidian conception by contrast with the Way of Ideas. First, Reid presupposes that the mind has an irreducible capacity for intentional conception in which our mental states are uniquely about specific objects. This faculty cannot be explained in terms of further non-intentional states. How intentional conception can be explained is mysterious—Reid is well aware of this. Yet if “we were unable to give any account how we first got the conception of power, this would be no good reason for denying that we have it” (Reid 2001, “Of Power,” 5). In a contrasting methodological move, Hume suggests that intentional content through ideas is built up from the operation of the laws of association—contiguity in time and space, resemblance and perceived causation—working upon impressions and images in the mind (see Treatise, 4;, 33;, 319; Enquiry 7.2.6, 74).

Conception takes as its intentional objects items with varied ontologies, including physical objects and propositions (EIP 1.1, 24–5) as well as fictional beings like Brienne of Tarth. The fact that conception takes physical objects as (direct) intentional objects marks a key contrast with the Way of Ideas since, on the Way of Ideas, conception, perception, judgment and belief are “only different ways of perceiving ideas in our own minds” (EIP 4.1, 298). (Note that, according to Reid, the Way of Ideas fails to adopt a proper faculty psychology since all mental powers—memory, perception, abstraction, etc.—only ever take ideas as intentional objects.) Considerable work in the secondary literature has been devoted to determine just what the content of conceptions are or can be.

The state of conception is not a ‘quantitative’ state for Reid, as are ideas according to Hume, who writes that some ideas are lively, others not, and others partially lively (Treatise 1.1.1). Hume uses these valences to determine which faculty is at work conceiving the idea. If I perceive an idea of a tree, my conception is lively and vivacious. If I remember the same idea of a tree, my conception is faint. Instead, Reid offers an account of a distinct faculty—conception—whose objects are embedded in other acts of the intellectual powers.

Reid's is an act-based theory of conception, in contrast to the Way of Ideas' object-based theory. Conceptions are ways of being aware of objects. To conceive of an object is to be aware of that object as the bearer of some particular property. So I can conceive that Brienne of Tarth is tall, or that Brienne of Tarth is female, or that Brienne of Tarth is a tall female. Being tall and being female are different properties, and their difference signals something important about Reidian conception. Being tall is a relational property since height is a trait that is assessed relative to some further thing or standard. To foreshadow, this subtle feature of conception will become important for Reid in his discussion of vision, especially his discussion of the relational property of visible figure.

2.2 Conception and Self-Knowledge

Reid affirms human beings know what are the intentional contents of their thoughts. If I think about a tree, then I am (defeasibly) justified that I am thinking about a tree. But the Way of Ideas jeopardizes Reid's commitment to the obviousness of our introspective ability to identify what we are thinking about. Like the more familiar veil of perception created by the representational role of ideas (see 3.1 The Way of Ideas and Representational Theories of Perception ), there is also a veil of conception. The seeds of Reid's argument are in Hume's candid remark that, given the Way of Ideas, mental events “seem entirely loose and separate. One event follows another; but we never can observe any tie between them. They seem conjoined, but never connected. ... [T]he necessary conclusion seems to be that we have no idea of connexion or power at all, and that these words are absolutely without any meaning” (Enquiry 7.2.6, 74; see Treatise, 67). Hume's causal explanation of reverberating impacts on impressions in the mind does not explain the content of our thoughts. Hume describes his theory of thought as corpuscular because impressions are atomic and separable mental states without any determinate intentional link to anything else. Note that Hume appears to be led to this position by way of his attempt to repurpose Newtonian mechanics, as it applies to bodies, to formulate a science of the mind.

Troubled by the statement from Hume, Reid constructs a reductio argument against Hume's theory of thought that plays upon the implications just noted. Reid presents this argument about the intentionality of thought through an analogy about the meaning of language, specifically, about someone who is blind to the meaning of the language in a book. Suppose I see unfamiliar ink marks on paper. I neither know what they mean nor even that they are meaningful to anyone. The meanings of these words are not intrinsic to the representations of the words, whether those representations are on paper or are sound waves, just like the intentional content of ideas is not intrinsic to the ideas themselves. Reid writes:

Suppose that ideas represent things like symbols; in this way, words and writing are known to express everything. Let the intellect, therefore, be instructed by ideas ... like a written or printed book, teaching us many things that are external, that have passed away, and that will come to be. This view does not solve the problem; for who will interpret this book for us? If you show a book to a savage who has never heard of the use of letters, he will not know the letters are symbols, much less what they signify. If you address someone in a foreign language, perhaps your words are symbols as far as you are concerned, but they mean nothing to him. (Philosophical Orations, Reid 1937, 35 & Reid 1989, 62)

Uninterpreted impressions are syntactic and purportedly representational. But “Symbols without interpretation have no value” (Philosophical Orations, Reid 1937, 35 and Reid 1989, 62). The resulting veil of conception implies that my impressions risk having no meaning for me. As a result, I do not know the contents of my own thoughts. Since this is absurd, Reid rejects the Way of Ideas' premise that leads to it. This fascinating argument has been explored and its affinities with John Searle's Chinese Room argument have been noted (Haldane 1989, 1993 & Nichols 2007, ch. 2), but it merits further development.

Reid's reductio of the Way of Ideas' theory of thought is intended to buttress Reid's claims that conceiving is irreducibly intentional and that, because of this, we have knowledge of the contents of our thoughts. Hume refrains from this claim in part because he sees no plausible metaphysics to support it. That is, if intentionality is endemic to thought, then the mind must not be what we think it is—a part of nature whose ideas operate on impact principles familiar from Newtonian mechanics. Reid is aware of this problem and prepares to bite the bullet. He describes our ability to apprehend one thing and not another as being “a natural kind of magic” (IHM 5.3, 59–60). In an effort to explicate this mysterious conceptual ability, Reid examines theories of thought of Aristotle and medieval philosophers. These theories posit substantial forms that conjoin thoughts to their intentional objects (Philosophical Orations, Reid 1937, 38 and Reid 1989, 66). Yet Reid judges them harshly and finds their ontology “incomprehensible” (EIP 2.8, 114). He withholds further speculation about the metaphysics that must undergird the claim that intentionality is primitive. This move illustrates a feature of Reid's philosophical method: common sense and knowledge are conceptually and explanatorily prior to metaphysical commitments.

2.3 Abstraction, Universals and General Conceptions

Reid is a nominalist about universals, which means that Reid does not believe that universals exist independently of the set of individual things that instantiate the shared property. (A universal is a property shared by many individuals, each of which instantiates the universal. In addition to opposing arguments for universals, he also opposes the form of nominalism advocated by Berkeley and Hume. Reid concludes that universals are general terms, not abstract or general ideas.

As usual, Reid builds his theory by learning from mistakes of earlier theories. Medieval philosophers argued, by Reid's account, that a universal exists independently of individual things. Some of these philosophers argued that the independent existence of a universal was existence as a form, a venerable philosophical term. Addressing this, Reid says it is impossible that “a triangle should really exist which has no precise proportion of sides and angles” and, generally, it is “impossible that any being should exist which is not an individual being” (EIP 5.6, 396). The principal philosophical error that led to the reification of universal properties as independent existents is the conflation of the objects of conception with the objects of perception (EIP 4.1, 302–3). The metaphysics required to undergird the existence of universals is anathema to Reid's commonsense commitments and his empirical method.

Next Reid considers a theory he associates with Locke, namely that universals do not exist as metaphysical forms but rather universals are abstract ideas. Abstract ideas arise through a process in which facts about time and space are stripped away from particular members of a class of thing leaving the abstract idea that represents common characteristics (EIP 3.2). Reid argues that there can be no such abstract ideas and that abstract ideas cannot represent members of a class of particulars (EIP 5.6, 391–3). Reid recognizes his great debt to Berkeley's criticisms of Lockean abstract ideas in these connections.

In effect, Reid arrives at his own theory by endorsing Berkeley's nominalism and attempting to remove from it reference to ideas. Reid replaces talk of abstract ideas with the term ‘general conception.’ As to the ontology of general conceptions, Reid says that human minds create them, not nature; this reconfirms Reid's nominalism about kind terms. General terms are the linguistic manifestation of general conceptions. General terms are general due to their referents, but they are particular token conceptions of individuals (EIP 5.2, 360). General terms refer to species or to attributes. When we conceive of general terms like ‘lion’ or ‘felony’ or ‘unicorn’, “the meaning of the word is the thing conceived” (EIP 5.2, 364). The term ‘lion’ does not resemble what it is about; it represents the class of lions by convention.

This leads to Reid's explanation of how general conceptions are formed. Reid says this typically happens via induction through testimony and not via definitions (EIP 5.2, 363). For Reid, forming general conceptions is typically a public process and a product of social practices, in contrast to Locke, Berkeley and Hume, for whom forming abstract ideas is typically a private process. (Though we do not enter into discussion of the role of testimony and social knowledge anywhere in this entry, it is especially prominent in Reid.) However, one may also form a general conception on one's own. In this case, in the first stage one analyses a subject into its attributes and names them, which Reid compares to a chemist analyzing a compound (EIP 5.3, 370). Next one observes that similar token attributes are possessed by many objects. Third is “combining into one hole a certain number of those attributes...and giving a name to that combination” (EIP 5.3, 365). The name that results may be the same as the name for a particular attribute, e.g. ‘whiteness,’ but used as a general term, ‘whiteness’ “implies no existence, but may be predicated of everything that is white, and in the same sense” (EIP 5.3, 367). This way of putting the point—that the general term whiteness may be predicated of everything that is white—has inspired a dispositional interpretation of Reidian general conceptions (Castagnetto 1992). Reid's writing on general conceptions and abstraction are areas—like the next topic, conception of non-existent objects—that have not been given much attention.

2.4 Non-Existent Objects

Reid believes that we can conceive of non-existent objects. This does not appear to comport with his alleged common-sense philosophy, creating an interesting conundrum for Reid interpreters. The best approach to understanding why Reid says we can conceive of non-existent objects is to put this in the context of Reid's opposition to skepticism. In the face of ‘veil of conception’ skepticism, Reid is intent on defending the transparency and self-knowledge of our thoughts. This leads him to argue that we can conceive of non-existent, fictional objects: “I conceive a centaur. This conception is an operation of the mind, of which I am conscious, and to which I can attend. The sole object of it is a centaur, an animal which, I believe, never existed” (EIP 4.2, 321; see EIP 2.12, 160). Suppose when I think I am conceiving of a centaur, I am conceiving of something else, like an image of a centaur. If so, then I do not know the contents of my thoughts. This yields skepticism about the contents of my thoughts, which Reid believes is false. So, by reductio he concludes that we are able to conceive of objects that do not exist.

The factual claim that I know what I am thinking appears to be obviously true insofar as anyone familiar with the meaning of ‘centaur’ conceives of centaurs when reading sentences that predicate properties of centaurs. Representational theories of thought like the Way of Ideas imply in this case that I do not conceive of centaurs but rather I conceive of mental representations of centaurs—images of them or words about them (EIP 4.2, 312). But Reid is asserting something different: I conceive of a centaur itself—not a mental representation of a centaur. Appreciating this point is necessary for understanding how strange Reid's theory appears to be.

Reid understands the Way of Ideas' error theory of the conception of a centaur: “The [proponent of the Way of Ideas] says, I cannot conceive a centaur without having an idea of it in my mind.... Perhaps he will say, that the idea is an image of the animal, and is the immediate object of my conception, and that the animal is the mediate or remote object.” Reid first responds that when he introspects he is aware of only one object of conception, not two. Second, the object of conception, he writes,

is not the image of an animal—it is an animal. I know what it is to conceive an image of an animal, and what it is to conceive an animal... The thing I conceive is a body of a certain figure and colour, having life and spontaneous motion. The philosopher says, that the idea is an image of the animal; but that it has neither body, nor colour, nor life, nor spontaneous motion. This I am not able to comprehend. (EIP 4.2, 321–2)

Even in difficult cases of thoughts about things like centaurs or circles (EIP 4.3, 323–4), their intentional contents are so transparent in a moment's reflection that we can identify them without mistake. Reid presses this epistemological point into heavy service in the context of the conception of non-existent objects. As with Reid's appeal to the irreducible intentionality of conception, Reid's position regarding the conception of non-existent objects has proven challenging to explain and to defend, but starts have been made (David 1985–6; Nichols 2002). Though his commitment to the importance of self-knowledge and the transparency of thought is obvious, Reid jeopardizes his allegiance to common sense by asserting we think of non-existent objects directly and without thinking of a mental intermediary.

3. Perception and Knowledge of the World

3.1 The Way of Ideas and Representational Theories of Perception

When discussing Reidian conception, we contrasted Reid's analysis of this faculty with the Way of Ideas'. For Reid, conceptions or ‘simple apprehensions’ possess intentional content in virtue of which these mental states are about other things. Conceptions can be about a wide variety of thing: I can conceive of pain, a shooting pain Descartes' foot, penne, and fictional characters like Podrick Payne. However, according to the Way of Ideas, I can conceive only of one type of thing: Ideas. According to the Way of Ideas, this holds true not only for conception but for all other faculties. If I tactilely perceive a pillow, my perceptual mental state is not about the pillow but rather it is about my idea or mental representation of the pillow. The basic structure of the representational theory of perception (and conception, memory, etc.) is explicitly endorsed by Descartes, Locke, Berkeley, Hume and many other Early Modern philosophers, as well as many living philosophers. We doubt that there is any other philosophical commitment that more exercised Reid's mind than this one, which he thought to be a source of ruin for many reasons. Both Reid and Richard Rorty see the Way of Ideas' representationalism as the key player in the story of Early Modern Western philosophy (Rorty 1981), which would continue as the story of contemporary analytic philosophy.

To the basic structure of the representational theory of perception, one of many components can be added to explain how we are ‘aware’ of external, physical objects even though the immediate object of perception is an idea. Here are three possibilities: (1) we are aware of external objects directly by perceiving representations of them, ideas; (2) we infer the existence and nature of external objects by perceiving ideas of them; or (3) there is no distinction between external objects and ideas, and, thus, when we perceive ideas we are perceiving external objects.

On option (1), the representationalist would need to offer an explanation of what it is that is so special about ideas that makes it the case that whenever we are perceiving an idea, we are directly aware of that which it represents. If this burden can be discharged, then it is possible to say that we are aware of external objects and remain consistent with the representational theory. In the history of philosophy Reid documents a series of failed efforts to explain how it is that perception of something in the mind could amount to any direct perception or awareness of some external object. These attempts make the mistake, he thinks, of giving unexplained and unexplainable powers to ideas: how could they possibly, all by themselves, attach our minds to objects whenever they are perceived? This question cannot be answered, so he thinks.

As to (2), Reid thinks that nobody who has absorbed Hume's lessons regarding causation would think that we can avoid skepticism about the external world while insisting that we infer its nature from the features of directly perceived ideas. On the representational model, external objects are the causes of our ideas. But if Hume is right about causation then we can only infer the nature of a particular unobserved cause of a particular observed effect when we have had repeated experience of conjunction of similar causes with similar effects. But according to the Way of Ideas we have never in fact had any experience of the relevant causes, namely physical objects and their qualities; we have only experienced their effects on our minds, ideas. Therefore, we can infer nothing about external objects by examination of the ideas which they cause in us. Option (2) appears to give skepticism a foothold.

As to (3), Reid believes that it amounts to an endorsement of idealism. Reid finds this theory implausible on its own, and in violation of a host of common sense principles, despite the fact that he admires Berkeley's efforts to pursue its full implications. Reid's arguments against representationalist attempts to recover the awareness of the external world through perception via options (1), (2) and (3) are by no means all of his arguments against the representationalist theory of perception or the Way of Ideas. We now introduce a few additional considerations for which Reid rejects that theory.

3.2 Reid's Arguments Against Representationalism

First, note that proponents of the Way of Ideas commit themselves not only to the existence of ideas but to pressing ideas into a number of functional roles in the mental lives of human beings. But, according to Reid, the underlying rationale for these commitments is typically not even stated by proponents of the Way of Ideas; it is most often taken for granted, or supported with demonstrably weak arguments. As a result, the rationale for commitments to an ontology of ideas and to their functional roles in the mind does not confer upon that commitment much epistemic justification. In the context of a passage in which Reid imagines a dialogue between “the vulgar” or the common person on the one hand and “the philosopher” on the other, Reid develops this reasoning as follows:

When, therefore, in common language, we speak of having an idea of anything, we mean no more by that expression, but thinking of it. The vulgar allow that this expression implies a mind that thinks, an act of that mind which we call thinking, and an object about which we think. But, besides these three, the philosopher conceives that there is a fourth—to wit, the idea, which is the immediate object. The idea is in the mind itself, and can have no existence but in a mind that thinks; but the remote or mediate object may be something external, as the sun or moon; it may be something past or future; it may be something which never existed. This is the philosophical meaning of the word idea; and we may observe that this meaning of that word is built upon a philosophical opinion: for, if philosophers had not believed that there are such immediate objects of all our thoughts in the mind, they would never have used the word idea to express them. (EIP 1.1, 31)

The Way of Ideas philosopher must talk the man on the street out of the belief that “When I perceive an apple in front of me, the apple is the very thing I'm perceiving.” The philosopher's response—that this is a mistake, that the object of perception is an idea of the apple—is predicated upon a rejection of the vulgar's conception of everyday thoughts about objects. But this rejection appears to rest in the opinion of philosophers rather than in the entailment of some as yet unstated but decisive argument. As a result, the rationale for commitments to an ontology of ideas and to their functional roles in the mind appears to Reid to be as dubious, if not more, than the man on the street's original commitment to a direct form of perception. If Reid is correct, then this shifts the burden of proof back upon the advocate of ideas.

Second, in a structurally similar argument, Reid observes something important about the intended explanatory power of the representational theory of perception. That theory is intended to explain the fact that our mental states manage to connect to real objects, manage to be about real objects. However, this fact about beliefs is only explained by the model if the model is less obscure than the phenomena to be explained by it. But, Reid observes, if we notice that we do not understand how it is that we manage to connect our minds to objects in the world, then it cannot help to say that we do this by first connecting our minds to mental representations unless we understand how it is that we manage to connect our minds to those mental representations. This consideration gives the following question its force: Why is the perception of a mental intermediary like an idea supposed to be more intelligible than the direct perception of a physical object? If it is no more intelligible, then the representational theory is not serving to explain what it was intended to explain. (See EIP 2.14, 185.)

In addition to burden of proof arguments against the Way of Ideas and the representational theory of perception, Reid also deploys a number of detailed arguments against specific components of these theories and against commitments they presuppose to be true. Though these arguments are unsuitable for recapitulation in the format of an encyclopedia entry, we report one such argument in brief in an effort to offer a hint at the robust detail, nuanced reasoning, and attention to physiological facts in Reid's theory of perception.

In one of a small handful of arguments that David Hume explicitly offers for a representational theory of perception, he concludes that “nothing can ever be present to the mind but an image or perception” (Enquiry 12.9; see Treatise, 187) by reasoning from considerations having to do with what is known as ‘perceptual relativity’. Hume writes, “The table, which we see, seems to diminish, as we remove farther from it: but the real table, which exists independent of us, suffers no alteration: it was, therefore, nothing but its image, which was present to the mind. These are the obvious dictates of reason” (Enquiry 12.9). This is perhaps the most compelling argument for the representational theory of perception.

Reid responds to this argument with several objections. First, Reid's analysis of each of the senses in Inquiry convince him that some senses do not represent physical objects in the same way that all the others do. Touch (IHM ch. 6), for example, presents qualities of objects to the mind in ways more direct than does vision (IHM ch. 7), resulting in what has been referred to as Reid's hybrid theory of perception. A conclusion generalized to all the senses (“nothing can ever be present...”) but based solely on one sense is therefore invalid.

The next of Reid's objections is more involved, requiring introduction of a key distinction Reid makes on the basis of his attention to the phenomenology of visual experience. First, though, notice Hume's use of the term ‘table’ and ‘the real table’ in the argument above. Following Berkeley's New Theory of Vision, Reid distinguishes between visible figure from tangible figure (IHM 6.2, 81–2), or later, which is virtually the same distinction, between ‘apparent magnitude’ and ‘real magnitude’ (EIP 2.14, 181). The tangible or real figure of an object is measured by yards or inches, but the visible figure is not. Instead, the visible figure depends for its geometric properties—the surface area that the table occupies in the visual field—upon properties of the tangible figure, including its shape and its distance from the eyes. Reid recognizes the obvious phenomenon of perceptual relativity, for example, that when I see a table at 10 yards and at 100, its “visible appearance, in its length, breadth, and all its linear proportions, is ten times less in the last case than it is in the first” (IHM 6.2, 81–2). The tangible figure is measured by different means, sensed by a different faculty, and is extended in three dimensions. This leads Reid to argue that “The ingenious author has imposed upon himself by confounding real magnitude with apparent magnitude,” that is, Hume equivocates (EIP 2.14, 182).

A pressing question arises at this juncture: Hasn't Reid now adopted a position according to which there are perceptual intermediaries in the visual perception process? If so, doesn't this imply that Reid's theory will share many of the foibles with which he charges the Way of Ideas? Most answers to these two questions have proven to be contentious in part because Reid's interpreters often bring to this issue prefabricated, a priori definitions of ‘intermediary’, ‘representation’ and ‘directness’. While Reid's theory of visual perception implies that visible figures are seen, visible figures are radically unlike ideas. Visible figure is not merely a representational intermediary; it is a relational property between physical objects and eyes of perceivers. Only because ideas lack these characteristics is Hume able to reason from the claim that mind-independent tables are not immediate objects of awareness to his stated conclusion above that we immediately perceive only ideas or representations. That is the first of two important points in response to the first question.

The second enters into Reid's development of a consistent geometry for visible space, perhaps the most technically brilliant piece of writing by any major British empiricist. This work allows Reid a crucial reply to advocates of representational theories. Reid discovered and mathematically described a law-like variation in the visible figure of an external object with intrinsic (shape, size) and extrinsic (distance, angle of orientation) properties of the tangible figure. Reid summarizes his conclusion writing:

[T]he real table may be placed successively at a thousand different distances, and, in every distance, in a thousand different positions; and it can be determined demonstratively by the rules of geometry and perspective, what must be its apparent magnitude and apparent figure, in each of those distances and positions.” (EIP 2.14, 183)

Hume assumes that the relationship between a visible figure and a tangible figure is subjective and mind-dependent. Not only is that erroneous, but, given Reid's work, the systematic variation of the visible figure with the tangible figure is in fact evidence for the objectivity of its independence from one's mind.

Reid further argues that this objective relationship between, for example, seeing the visible figure of a coin and conceiving of the coin becomes automated over time through repeated experience. Perhaps the first time I saw a coin it was presented to me at an angle, its visible figure an ellipse. I might mistakenly form a perceptual belief about an elliptical object, even though the coin is circular. Over time, this process is habituated through a process that Reid describes with the help of a distinction between original and acquired perceptions. In short, my faculty of visual perception, if given experience, will successfully model the geometrically demonstrable relationship between visible figure and tangible figure.

This family of arguments has received considerable attention due to a number of its complexities. These involve attempts to answer questions including the following: As we have seen, through his science of perspectival shapes of objects Reid argues that they are geometrically equivalent to shapes projected onto surfaces of spheres. What is Reid's formal proof for this, and is it valid (Yaffe 2002)? Does Reid offer the first recorded non-Euclidean geometry in the history of mathematics (Daniels 1972)? What is the bearing of Reid's repeated attempts to derive Euclid's parallel postulate from the axioms of incidence on an interpretation of the geometry of visible space as non-Euclidean (Grandi 2005)? Where does Reid's theory of perception and geometry of visibles leave the ontology of visible figure (Nichols 2007, ch. 4)? In what ways is visible figure the object of visual perception, and in what ways not (Yaffe 2003; Falkenstein and Grandi 2003)? Does Reid's geometry of visible space jeopardize his theory's ability to avoid a representational theory of visual perception (Van Cleve 2002)? Does visible figure and awareness of it preclude non-inferential perceptual knowledge from vision, or rather, what is the relationship between original and acquired perceptions and visible figure (Nichols 2007, ch. 8)?

Now that we know how and why Reid argues against the Way of Ideas' representational theory of perception, we needn't address these questions in order to continue with a discussion of the components of Reid's own more direct theory of perception. Here we need to keep in mind the fact that, though contemporary philosophers write a priori about perception, sensation, and knowledge, Reid does not offer necessary and sufficient conditions for perception or for its component processes. Consistent with his Newtonian empiricism, Reid is aiming for something else altogether: observations and accurate experiments that reduce to general rules (EIP 1.3, 49–50; EIP 2.8, 120–1). Though operations of our intellectual powers are not definable or analyzable a priori, it is possible to describe the operations Reid has in mind.

According to Reid, the perceptual process operates as follows. Individuals with functioning sensory organs and a developed brain interact with physical objects in the world. These objects cause sensory experiences in individuals. These sensations function as natural signs for qualities of the objects. The experience of a sensation orients my cognition so as to form a conception of a quality of a mind-independent object. Sometimes, though rarely, an individual's perception of an object will not only cause a sensation but will also cause the person to become aware of the sensation—perhaps even at the exclusion of any further awareness of primary qualities of the object. For example, when I get hit by a baseball thrown 90 mph I focus on my pain sensations, not the qualities of the ball. To perceive an object is to be aware of the object or its quality in a particular way, as the possessor of a particular quality, and, at the same time, to be convinced that the object exists and is as it is conceived it to be. To defend a common sensical theory of perception that is supported by observation and experiment, and that is capable of delivering knowledge of real objects, Reid thinks he needs to show that we are directly aware of real objects, in contrast to key features of the Way of Ideas' representational theory of perception just discussed. While there is debate over the precise sense in which, for Reid, we are directly aware of objects, this much seems clear: whatever the sense of “direct” is in which the subscribers to the representational theory take us to be directly aware of ideas, it is in that sense that Reid takes us to be directly aware of real objects.

3.3 Sensations as Natural Signs of Qualities

In addition to being a Newtonian empiricist, Reid is an expert phenomenologist, acutely aware to finely grained features of our experience, especially our sensory experience. When we touch a table, while we conceive of it and often form beliefs about it, we also sense it. (See 2.1 Conceiving) The immediate effect that objects have on us is to cause sensations. Sensations are always associated with a particular organ of sense; they are always distinctly of, for instance, touch or vision. Sensations are the feelings that are the immediate mental causal consequences of the influence of objects on us. We become aware of the qualities of objects following the sensations that those objects cause. However, for Reid, the conceptions of objects that follow from our sensations are not derived from our sensations since they do not bear any kind of resemblance to the qualities which cause them. Awareness of sensations are not, for Reid, essential intermediaries for formation of perceptual beliefs. So what is the precise relationship between sensory experiences and conceptual content? This is a delicate question for Reid since the directness of his theory of perception depends upon advocacy of a physiologically and phenomenally real theory that accounts for our experience while avoiding theoretical pitfalls that risk dragging his theory much closer to a representational theory than Reid would like.

As we have often in this entry, we begin with historical context since Reid's theory of the relation between sensations and qualities will appear stranger than it is without background. According to the Way of Ideas, ideas and images are mental representations in an individual's mind, and my perceptual awareness of a table depends upon my prior awareness of these ideas of sensation. This veil of perception is why advocates of the Way of Ideas struggled to provide epistemic justification for beliefs about external, mind-independent, qualities of physical objects, and avoid skepticism. But besides the epistemic problem, advocates of ideas faced a difficulty accounting for the origins of the contents of our very ideas of external objects. Though not easy to appreciate, this problem it is quite important.

Reid argues that advocates of the Way of Ideas do not have a plausible solution to this problem. Hume says bluntly that “[E]very simple idea has a simple impression, which resembles it” (Treatise, 3), which is a statement of what is known as the Copy Principle: ideas have the content that they do because they are copies, and resemblances, of impressions. Think of a table. For Hume, your thought of a table is derivative and copied from a sensory impression that you have had of that very table. The explanation of the content of the idea of that table thus concerns the contents of your simple impression of the corresponding physical table. The Copy Principle is an exceedingly powerful tool on behalf of the Way of Ideas for a variety of reasons, but Reid is unimpressed with the evidence on its behalf, especially as presented by Hume in Treatise. Reid's response to Hume leads to an argument, which we can call his ‘Sensory Deprivation Argument’.

Reid's Sensory Deprivation Argument proceeds as follows. First, Hume's stated justification for the Copy Principle is inductive: he challenges people to find an idea that is not derived from a sensory impression, after he says that it appears all his ideas are copied from sensory impressions. But that, says Reid, is an exceedingly weak justification (EAP 1.4, 23). Besides, Hume's claim that the principle is “certain” is mistaken because the argument he sets out for the principle is inductive (IHM 5.7, 69–70, 75–6). After those ground clearing moves, Reid begins the argument proper with a thought experiment. Exhibiting affinities with Condillac's remarkable Treatise on Sensations (1754), Reid asks his readers to imagine a blind adult subject. Not only has he lost his sight, but he has “lost all the experience, and habits, and notions he had got by touch; [he lacks] the least conception of the existence, figure, dimensions, or extension, either of his own body, or of any other; but to have all his knowledge of external things to acquire anew, by means of sensation, and the power of reason, which we suppose to remain entire” (IHM 5.6, 65). Reid imagines introducing this blind experimental subject to a number of tactile sensations, one by one, from the most simple to the most involved: a single pin prick in a split second of time; a blunt object pushed against a small surface area of skin; the same object pushed against skin over a period of time “with a force gradually increased till it bruises him”; an object covering a large surface area of skin, applied over a period of time, and so on. Reid's emphasis on tactile sensations is important to understand: he uses this running example because ideas of primary qualities are the most important for gaining knowledge of the world and are the ideas essential for any science.

Once a handful of skin cells are stimulated for the briefest of moments by a pin, the subject has received a sensory impression—or in Reid's language, he has had a sensation—but, Reid argues, this does not equip the individual to formulate an idea of extension or figure, space or motion, all primary qualities of the pin. He writes, “[The primary qualities] have no resemblance to any sensation, or to any operation of our minds; and, therefore, they cannot be ideas either of sensation or of reflection” (IHM 5.6, 67). For each subsequent iteration of tactile sensations, Reid argues that those sensations too are incapable of delivering to the subject ideas of extension, figure, space or motion. His conclusion has two parts. First, the Way of Ideas's story about the relation between sensations and the intentional contents of conceptions is mistaken. Second, with an eye toward developing his own theory of sensations, he writes,

[T]his connection between our sensations and the conception and belief of external existences cannot be produced by habit, experience, education, or any principle of human nature that hath been admitted by philosophers. At the same time, it is a fact that such sensations are invariably connected with the conception and belief of external existences. Hence, by all rules of just reasoning, we must conclude, that this connection is the effect of our constitution, and ought to be considered as an original principle of human nature, till we find some more general principle into which it may be resolved. (IHM 5.3, 61/122b)

The process by which sensations give rise to our conceptions of objects is something Reid calls ‘suggestion’. Sensations in this process he calls ‘signs’. The qualities of objects are ‘suggested’ by our sensations when they function as signs, so when we have sensations we come to be aware of those objects as possessing those qualities. But what is suggestion supposed to be? The first thing to notice is that the suggestion relation or the ‘sign-signified’ relation is contingent. Whereas philosophers prefer necessities, Reid does not believe the messiness of the actual perceptual process contains any necessity at this juncture. Second, suggestion is a pseudo-linguistic notion for Reid. Signs suggest conceptions of that which they signify. The word “dogs”, for instance, leads those who are familiar with the word to think of certain domesticated animals who are also man's best friends.

But for those who already know the term, this does not happen by hearing the word “dogs” then deriving or inferring some object that has some peculiar fitness to the word. The relation of signs to things they signify in spoken and written language is almost always arbitrary in the sense that there is no similarity between dogs and the word “dogs”. In this sense, “dogs” is an artificial or artifactual sign, for Reid, in contrast to natural signs. Blushing signifies one of a small number of mental states because blushing is caused by events of embarrassment, anger, or romantic stimulation. Blood rushing through the subcutaneous circulatory system in the face reliably occurs, across members of our species, in response to a small set of stimuli with apparent universality, making blushing a natural sign. We discover this relationship by observation and experience.

However, according to Reid, a subset of natural signs lead us to think of what they signify without any prior experience. Members of this category “which, though we never before had any notion or conception of the things signified, do suggest it, or conjure it up, as it were, by a natural kind of magic, and at once give us a conception, and create a belief of it” (IHM 5.3, 60). Put your hand on a table, pause and experience the sensation of hardness. This tactile sensation leads us immediately to conceive of that which caused it as being hard, as having a certain resistive construction. But we are aware of the table's hardness, which caused the sensation, automatically. We do not understand why it is that we think of this special kind of physical constitution after having this kind of tactile sensation. Nonetheless, we must “conclude, that this connection is the effect of our constitution, and ought to be considered as an original principle of human nature” (IHM 5.3, 61). This is the defining feature of the kind of natural signs of which the tactile sensation of hardness is an instance: these signs lead us to conceive of what they signify simply because we are built in such a way as to have such conceptions on encountering such signs. Reid's approach to this relationship contrasts sharply with that taken by most advocates of the Way of Ideas, who attempted to posit a strong relationship between ideas of sensation and qualities of physical objects.

Placing my hand on a table, my tactile sensation immediately leads my mind to a conception or apprehension that I am touching a hard object. When does one's conception of an object as having a particular quality amount to a perception, a conception accompanied by a conviction of its accuracy? The answer is: when one comes to have that conception because one has encountered a natural sign which leads one to the conception, and that natural sign leads one to the conception merely because of one's constitution. Conviction in the accuracy of a conception is bestowed on the conception, Reid thinks, just when the relevant conception comes about because of our nature or constitution. When we conceive of an object in a particular way merely because it is in our nature to conceive of the object that way, then the conception is non-optional, unavoidable, and is thus one that we cannot help but trust. Perceptions, then, are dictates of common sense: to be aware of an object in perception is to have a belief which you cannot give up given your constitution.

Reid believes that our minds come to be connected to the mind-independent world in something like the way that we come to grasp objects through a language designed for the purpose. However, when we come to be aware of objects through our senses, we do so by utilizing something like a language embedded in our constitutions: our sensations function like a language that nature has constructed. While it is in a sense only a metaphor to say that we know about the world because the world speaks to us, it is a metaphor that illuminates the facts as Reid sees them.

3.4 Primary and Secondary Qualities

Intertwined with Reid's arguments that we can be immediately aware of objects through their relational properties—such as their apparent magnitudes—are Reid's arguments regarding the distinction between primary and secondary qualities. As we would expect, Reid approaches this distinction as a scientist of the mind rather than as a metaphysician, and, as we would also expect, Reid's entrance into this terrain is paved with references to earlier thinkers.

Locke thought that our ideas of “primary qualities”—shapes, sizes, motions, textures and physical construction—resemble the qualities which cause them. However, according to Locke, our ideas of a variety of other qualities—colors, sounds, tastes and smells—do not. Ideas of colors, sounds, tastes and smells, or “secondary qualities,” are caused by certain complex configurations of primary qualities that bear no resemblance to the ideas which they cause. Reid is deeply struck by Berkeley's attack on this distinction at Principles of Human Knowledge 1.9–15. He agrees with Berkeley that no mental state or object could possibly resemble anything that is not, itself, a mental state or object. In Berkeley's famous words, “An idea can be like nothing but an idea” (Principles of Human Knowledge 1.8). Mental states and objects have only mental properties, but only something that is, itself, a mental state or object can have a mental property. Hence, nothing can resemble—that is, share a property in common with—a mental state or object other than another mental state or object. Berkeley took this point to show that no non-mental cause of an idea could resemble it, whether the relevant idea were an idea of a shape or a color. Berkeley concludes that Locke's distinction between primary and secondary qualities is a grand mistake.

Reid accepts, for roughly Berkeley's reasons, that sensations cannot possibly resemble their causes—a fact that Reid deploys in his Sensory Deprivation Argument discussed above. Further, he accepts Berkeley's objections to Locke, and takes them to show that no mental events or states, whether sensations or the conceptions of objects that follow them, could possibly resemble any non-mental object. Another reason that Reid cannot draw the distinction between primary and secondary qualities in the way that Locke did is that he denies that conceptions of objects which we have following sensory experiences are to be analyzed as perceptions of ideas. Instead, these conceptions function to make us aware of qualities in objects that caused the sensations. All of this would make it seem that Reid would simply side with Berkeley and deny that there is any important difference between primary qualities and qualities like colors, sounds, tastes and smells. Instead, Reid defends the distinction between primary and secondary qualities on grounds quite different from Locke's, grounds that he takes to be immune to Berkeley's criticisms of the distinction.

Reid accepts that the qualities which we ordinarily conceive objects to have—whether shapes, sizes and motions, on the one hand, or colors, sounds, tastes and smells, on the other—are genuinely possessed by those objects (barring illusions and disorders of various sorts, which are, incidentally, difficult for any direct theory of perception like Reid's to explain). He thinks that shapes, sizes and motions are intrinsic properties of objects while colors, sounds, tastes and smells are relational properties of objects. But neither secondary nor primary qualities resemble the sensory experiences that they immediately cause in us. Colors, sounds, tastes and smells are powers to produce certain characteristic sensations in us in normal conditions. To believe that a rose has an agreeable smell or a red color is not to perceive any intrinsic qualities of the object, but is, rather, to perceive that the object bears a certain relation to something else—our minds. Reid writes,

The object of my perception, in this case, is that quality in the rose which I discern by the sense of smell. Observing that the agreeable sensation is raised when the rose is near, and ceases when it is removed, I am led, by my nature, to conclude some quality to be in the rose, which is the cause of this sensation. This quality in the rose is the object perceived; and that act of my mind by which I have the conviction and belief of this quality, is what in this case I call perception. (EIP 2.16, 194)

Physical structures of the rose in front of me—the anther in the rose's stamen—produce pollen, which has a certain molecular composition that results in my sensation of its odor. Other molecules in the petals of the rose reflect light at a certain wavelength which in turn causes in me a certain characteristic visual sensation of red. If I am speaking correctly when I say “That rose smells good”, I am reporting the fact that I conceive of the rose as possessing a particular relational property: I am aware that the rose has the property of being-such-as-to-cause-in-me-sensations-of-a-sweet-smelling-odor. Ultimately, the rose possesses this relational property because of facts about its molecular structure that account for its producing this odor in a certain way, and facts about me that account for the fact that these pollen molecules enter my nasal cavity, eventually reaching the olfactory bulb, and cause certain sensations in my mind. But when I am aware of the odor of the rose, I am aware of none of that; I am aware only of the fact that there is something about the rose that makes it cause in me certain sensations in normal conditions.

Our conceptions of qualities such as smells and colors are to be contrasted with our conceptions of primary qualities or configurations of primary qualities, such as hardness. Say I'm holding the rose's stem in my hand while I'm looking at it. I am having two importantly different sensations: a visual sensation of red, and a tactile sensation of hardness. For Reid, neither sensation resembles anything in the object; both give rise to conceptions of the object as possessing certain properties. The visual sensation gives rise to a conception of the object as possessing a particular relational property: its power to produce certain sensations in me in normal conditions. The tactile sensation gives rise to a conception of the rose's stem as possessing a particular intrinsic property: the complex configuration of primary qualities that is hardness.

For both Locke and Reid, we are aware of objects as they are intrinsically only when our awareness is caused by the primary qualities of objects. But for Reid, and not for Locke, we are genuinely aware of objects as they are when our awareness results from the secondary qualities of objects; but we are aware of those objects only as they are relative to us, and not as they are in themselves.

“When a primary quality is perceived, the sensation immediately leads our thought to the quality signified by it, and is itself forgot.” (EIP 2.17, 204)

“[T]here appears to be a real foundation for the distinction; and it is this—that our senses give us a direct and a distinct notion of the primary qualities, and inform us what they are in themselves. But of the secondary qualities, our senses give us only a relative and obscure notion. They inform us only, that they are qualities that affect us in a certain manner—that is, produce in us a certain sensation; but as to what they are in themselves, our senses leave us in the dark.” (EIP 2.17, 201)

“[T]he sensations belonging to secondary qualities are an object of our attention, while those which belong to the primary are not.” (EIP 2.17, 204)

Secondary and primary qualities are characterized by their relations to the mind perceiving them and to the physical qualities in objects that they signify. Primary qualities stand out as qualities of which we have “direct” and “distinct” notions. This is not so for secondary qualities. To return to Reid's rose, he writes of the smell of the rose that

I have a distinct notion of the sensation which it produces in my mind. ... The quality in the rose is something which occasions the sensation in me; but what that something is, I know not. My senses give me no information upon this point. The only notion, therefore, my senses give is this—that smell in the rose is an unknown quality or modification, which is the cause or occasion of a sensation which I know well. The relation which this unknown quality bears to the sensation with which nature hath connected it, is all I learn from the sense of smelling; but this is evidently a relative notion. The same reasoning will apply to every secondary quality. (EIP 2.17, 202/314b)

Reid does defend a primary/secondary quality distinction, but, unusually, Reid's distinction is drawn not in metaphysical terms but in epistemic terms premised upon discussion of the differing ways that secondary and primary qualities relate to our minds (Nichols 2007, ch. 6).

An important note at this juncture is that Reid very infrequently uses contemporary epistemological terms such as “knowledge”, “rational”, or “justified”, and even when he does, he does not offer necessary and sufficient conditions for their definition. Reid says above that sensations caused by primary qualities “inform us” of the qualities in the objects that caused them. One ought not look to Reid's epistemology for a detailed formal apparatus clearly identifying his externalism about justification, for example, even though he can be credited with bringing what we now call externalism about justification into Early Modern philosophy.

3.5 Responding to Skepticism about the External World

We are now in a position to understand the force of Reid's most important response to any argument purporting to show that the external world either might not exist, or might not be anything like the way we take it to be. In one canonical statement of his position, Reid says,

The sceptic asks me, Why do you believe the existence of the external object which you perceive? This belief, sir, is none of my manufacture; it came from the mint of Nature; it bears her image and superscription; and, if it is not right, the fault is not mine: I even took it upon trust, and without suspicion. Reason, says the sceptic, is the only judge of truth, and you ought to throw off every opinion and every belief that is not grounded on reason. Why, sir, should I believe the faculty of reason more than that of perception?—they came both out of the same shop, and were made by the same artist; and if he puts one piece of false ware into my hands, what should hinder him from putting another? (IHM 6.20, 168–169)

The mistake that the skeptic makes, according to Reid, is to deny the truth of something that is demanded by our constitutions. To perceive an object as possessing a particular property is to have a conception of the object delivered by one's constitution. What makes us convinced of the accuracy of the conceptions of objects involved in perception is that they arise from our constitutions. But, asks Reid here, why do we find skeptical arguments so compelling? Why would someone sincerely accept the radical skeptical conclusions following from Descartes's hypothesis of the evil demon?

Ultimately, we think that such arguments lead to their conclusions because we accept certain logical principles—such as the law of non-contradiction, or modus ponens—which appear to us to be self-evident. But to say that such principles are self-evident is just to say that we cannot help but accept them. It is not to offer any non-circular justification of those principles. But the irresistibility of a belief is a very good indicator, Reid thinks, that we hold that belief merely because of the way we are built, merely because of our constitution. But then the skeptic has merely placed the skeptical conclusion on the same footing as the common sense belief about the external world: both rest on something that we are compelled to believe by our constitutions. However, in order to overthrow common sense, the skeptic must place the skeptical conclusion, rather, on a firmer footing than the common sense conclusion. Thus, the skeptic gives us no reason to reject common sense beliefs about the external world. Reid has defended common sense through construction of a direct theory of perception that avoids the pitfalls of the Way of Ideas' representational theory. Rather than landing in the “coal pit” of skepticism, as Reid calls it, his theory purports to deliver knowledge of the external world.

4. Causation and Free Will

4.1 Objecting to Hume

Reid develops his account of causation in light of Hume's account. As Reid sees it, Hume starts with the assumption that if we are to learn what causation is, we must first determine from what aspect of our sensory experience the concept of causation is derived. Hume would put the point in terms of the Way of Ideas as follows: we must determine from what impression our idea of causation is copied. However, Hume believed he had discovered that there is nothing in our sensory experience corresponding to our ordinary notion of the causal relation. In an inversion of the empiricist method attributed to Reid above, Hume believed that causes necessitate their effects, then he followed this commitment by arguing that we lack sensory awareness of this necessitation. We see the first billiard ball hit the second. We see the second move. We do not see the movement of the first assure, or necessitate, the movement of the second. So Hume concludes that causation must be something different from what we take it to be ordinarily.

But what is it? To answer this question, we must determine from what sensory experiences we derive the idea of causation. It turns out, as Reid reads Hume, that the sensory experiences that give rise to our idea of causation are sensory experiences of what Hume calls constant conjunction. The heating of the water is regularly followed by the water's boiling. To say that the one event causes the other is just to say that the two events always co-occur, or that there is a brute law linking them. This is not to say that the first event necessitates the second. The necessitation that we ordinarily take to be involved in causation is not in the world; it is in our heads as an expectation arising due to previous experience.

Reid accepts much of the negative side of Hume's view of causation while rejecting Hume's assertions of the import of those negative discoveries. Reid agrees, that is, that we have no sensory experience of the necessitation of an effect by its cause. But this does not imply our ordinary concept of causation is mistaken. Instead, Reid thinks, the concept of causation is not an idea copied from a sensory impression in the first place. Reid's complex thoughts here are tied to perception and the role of sensations in perception. (See 3.2 Reid's Arguments Against Representationalism, above.) For him, sensations never bear resemblance to the qualities they help us to perceive. It is hardly a surprise, then, that the sensory experiences from which our thoughts about causation spring bear no resemblance to the causal relation. But this fact no more undermines our concept of causation than the lack of resemblance between our sensory experiences of ordinary physical objects and those objects themselves undermines our thoughts about such objects. Like our thoughts about objects, our thoughts about the causal relation are genuinely about the causal relation, despite the fact that the sensations from which those thoughts spring, i.e.the sensations of the relation of constant conjunction, bear no resemblance to the relation they lead us to think about. We have no sensations resembling necessitation, and, yet, causes necessitate their effects.

However, Reid also offers criticisms of Hume's view of causation that can be accepted independently of the Reidian view of perception and sensation. Two of his most influential criticisms are of Hume's view that our ordinary concept of causation is reducible to the relation of constant conjunction.

Of the first he writes, “[I]t would follow from [Hume's definition of causation], that whatever was singular in its nature, or the first thing of its kind, could have no cause” (EAP 4.9, 250). Reid's point is that if the relation of causation is really that of constant conjunction, then the first time that two types of event are conjoined, the first cannot be the cause of the second. If there is no history of conjunction, there is no causation. It would seem to follow from Hume's definition, for instance, that if an earthquake razes Mexico City, and no earthquake has ever done so before, then the earthquake is not, in fact, the cause of the city's fall, an absurd implication. This objection might not show Hume's theory is mistaken—we omit consideration of it further here—but it does pose a question with which any Humean must wrestle: Which constant conjunctions are the genuine ones on the basis of which the causal relation can be said to hold, and which are not? The more specifically any two events are described, the more likely that there will be no history of conjunction of the relevant sorts of events. But what degree of specificity or generality in description is the right degree?

The problem is made clearer by Reid's second objection to Hume's analysis of causation and constant conjunction. He writes, “It follows from [Hume's] definition of a cause, that night is the cause of day, and day the cause of night. For no two things have more constantly followed each other since the beginning of the world” (EAP 4.9, 249). Since we don't ordinarily think that day is the cause of night, or vice versa, Hume must deny that the two are actually constantly conjoined, or, rather, he must insist that the constant conjunction between the two of them is not of the right sort for the relation between them to be one of causation. Hume thinks that there is causation between two events just in case there is a law linking them and also thinks that there is a law linking two events just in case they are constantly conjoined. Reid's first objection shows that it is not the case that wherever there is no constant conjunction there is no law. His second objection shows that it is not the case that wherever there is constant conjunction there is a law. Constant conjunction is neither necessary nor sufficient for the presence of a genuine law. The hard problem that remains for the Humean, then, is to produce criteria for distinguishing genuine laws from regularities that are not laws at all.

These criticisms lead Reid to hold that there is a legitimate causal relation between two events whenever the two are conjoined by a law of nature, even if laws of nature do not simply amount to constant conjunctions. He uses the term physical causation to refer to the relation that holds between two events just in case they are conjoined by natural law. He takes the discovery of the physical causes of phenomena to be central both to the sciences and to ordinary life (see IHM 6.12, 122; EAP 1.5, 28–9; EAP 4.3, 211–2). But he also holds that genuine causation, what he calls efficient causation, is not reducible to physical causation. The reason is that, for him, a law of nature is not a brute conjunction between events. Rather, it is a regularity in the behavior of the efficient cause of observed phenomena. Consistent with remarks above (see 1.2 Newtonianism and Empiricism), Reid writes,

[S]upposing that all the phenomena that fall within the reach of our senses, were accounted for from the general laws of nature, justly deduced from experience; that is, supposing natural philosophy brought to its utmost perfection, it does not discover the efficient cause of any one phenomenon in nature. The laws of nature are the rules according to which the effects are produced; but there must be a cause which operates according to these rules. The rules of navigation never navigated a ship. The rules of architecture never built a house. (EAP 1.6, 38)

It is a law that unsupported objects close to the earth will fall. What this means, Reid thinks, is that the physical cause of the fall of an object regularly causes objects to fall when they are unsupported. Physical causation, for him, is parasitic upon the more basic kind of causation, namely efficient causation. But what is efficient causation? Reid writes, “In the strict and proper sense, I take an efficient cause to be a being who had power to produce the effect, and exerted that power for that purpose” (Letter to James Gregory, in Correspondence 174). Efficient causation is necessitation of the sort that Hume thought not to exist. To efficiently cause an event is to be capable of seeing to it that the event occurs, and to make an effort to see to it that it does. Thus, at the bottom of Reid's theory of causation are his notions of power and exertion.

4.2 Power, Exertion and Moral Liberty

Reid is clear that power is not the sort of thing that admits of a logical definition. We cannot reduce it to some set of simpler qualities. However, this doesn't mean that we can't say anything about what power is. On the contrary, Reid makes a variety of claims about power. Importantly, he claims that power is the quality that, when coupled with exertion, necessitates a particular effect. Taking a cue from ordinary language, he holds that it is a contradiction to say that an entity has the power to do something, and exerts that power, and yet the effect fails to come about. If the effect fails to come about when the entity exerts itself, we might say, what that shows is that the entity only seemed, but didn't actually, have the power to bring about the effect. Reid also claims that any agent who has the power to do something also has three other inter-related powers: the power not to do the thing, the power to try to do that thing (that is, the power to exert his power to do the thing), and the power not to try to do that thing. He claims further that any agent who has the power to do something must believe himself to have that power.

Any one of these claims can be questioned, even if Reid is right that they are embedded in our ordinary concept of power. Take, for instance, the claim that an agent who has the power to do something also has the power not to do it. Locke offers the following example, which seems to be a counterexample to this claim:

[S]uppose a Man be carried, whilst fast asleep, into a Room, where is a Person he longs to see and speak with; and be there locked fast in, beyond his Power to get out: he awakes, and is glad to find himself in so desirable Company, which he stays willingly in, i.e.

UC Davis Philosophy 22

Reid Lecture Notes

G. J. Mattey

Revision of May 20, 2009

Thomas Reid was one year younger than David Hume and was Hume's most important contemporary critic. In a series of three books, Reid laid out a comprehensive theory of the human mind, the scope of which rivaled or even surpassed that of Hume's theory. In his first book, Inquiry into the Human Mind on the Principles of Common Sense (1764), Reid described in great detail the workings of the senses. His second book, Essays on the Intellectual Powers of Man (1785), extended this account to all human cognitive powers. The third book, Essays on the Active Powers of the Human Mind (1788) looks at the conative side of the human mind, discussing the will, the principles of human action, human liberty, and morality.

Reid was traditionally known primarily as one of a group of anti-Humean Scottish philosophers of "common sense," along with such lesser lights as James Oswald and James Beattie. Kant criticized these philosophers particularly harshly (Prolegomena to any Future Metaphysics, Preface). Reid's reputation suffered along with that of his fellows, and it is only in the last half of the twentieth century that he has finally emerged as a philosopher to be taken seriously.

Until fairly recently Reid was best known for his account of the history of modern philosophy. According to Reid, Descartes replaced the ancient account of mind with a new one that contained the seeds of its own destruction. After Descartes, philosophers who adopted his main priniciple became more and more skeptical: Locke was more skeptical than Descartes, Berkeley than Locke, and Hume embraced universal skepticism. (See, for example, Inquiry, Conclusion.)

Although Reid was critical of all of these philosophers, he reserved his harshest criticism for Hume, while at the same time professing his admiration for Hume's logical powers. Writing of the system of the Treatise in the Essays on the Active Powers of the Human Mind, Reid declared, "That system abounds with conclusions the most absurd that ever were advanced by any philosopher, deduced with great acuteness and ingenuity from principles commonly received by philosophers" (Essay I, Chapter 2). Reid summarily dismissed Hume's attempt to produce a positive account of the human understanding.

It seems to be a peculiar strain of humour in this author, to set out in his introduction by promising, with a grave face, no less than a complete system of the sciences, upon a foundation entirely new--to wit, that of human nature--when the intention of the whole work is to show, that there is neither human nature nor science in the world. (Inquiry, Chapter I, Section V)
This view of Hume's Treatise was prevelant until the middle of the twentieth century, when commentators began to take Hume's "system of human nature" more seriously.


We begin our discussion of Reid with his account of the descent from the Cartesian theory of ideas to Humean universal skepticism, as Reid recounts it in the Conclusion of the Inquiry. (For the version found in the 1785 Essays on the Intellectual Powers of Man, click here.)

According to Reid, there are two ways of forming a conception of the human mind.

  • By reasoning analogically from known properties of material things
  • By directly observing the operations of the mind
As Reid saw it, ancient philosophy was in the grips of the first way of understanding the mind. So the mind was described as a breath or "pneuma" that pervades the human body and gives it life. The interaction between the mind and material things was modeled on the interaction between material things alone. Just as a seal makes an impression on wax, material things make an "impression" on the mind when it perceives them. This is the precursor to the view of Descartes that the mind is furnished with "ideas." Understanding the mind by analogy with material things promotes the view that the mind itself is material. One modern philosopher who described the human mind materialistically was Thomas Hobbes, for whom the mind is no more than an elaborate calculating device.

Reid credited Descartes with having broken free of analogical reasoning, at least to some extent. Descartes claimed that the mind is entirely distinct from the body, and that any "impressions" or "images" of objects are to be found in the body, not in the mind. When Descartes looked into his own mind (thus conceiving the mind properly), he found that he was essentially a thinking thing, a being endowed with understanding and will, both of which are not material.

But that is not the end of the story. For along with understanding and will, Descartes found his mind to be endowed with a faculty he called "sense" (though this was not strictly part of his essence). Sense is a passive faculty. When it senses, the mind is imprinted with ideas which are not the product of its will.

And here it is that Descartes made his fatal error. His description of the faculty of sense was the product of analogical reasoning, not of direct observation. The "ideas of sensation" are not literally images, such as those spread out upon the retina, but they are understood by analogy with images. And these alleged "ideas of sensation" form an impenetrable barrier between ourselves and the physical world.


Descartes himself did not reap the whirlwind that he had sown. Ultimately, ideas of sensation are treated by Descartes as being mere aids that allow us to avoid what harm us. The real knowledge of the world is gained through innate ideas, most notably that of God. The British philosophers cast off this safety net and wagered their fortune on experience alone. For Locke, all the materials on which the mind bases its judgments about the world beyond it are delivered through sensation. Locke argued that some of the ideas of sensation, the ideas of secondary qualities, do not resemble the qualities in the bodies which cause them. But Berkeley proved that the same reasoning applies to the ideas of primary qualities. His conclusion, according to Reid, followed inevitably from Locke's starting point: the only being we can attribute to bodies is being perceived.

Hume completed the descent into the absurd. Locke had held that our knowledge of the mind is gained through "ideas of reflection," as if the mind impressed images of its operations upon itself. Berkeley had realized that accepting Locke's view would leave him just as skeptical about the mind as he was about external bodies. So he backed away from the Lockean claim that we know all objects only through ideas and allowed that we can have knowledge of one's own mind in a way that does not require ideas as intermediaries. Hume, on the other hand, had no fear of skepticism and embraced the consequence that Berkeley wanted to avoid, that minds, just like bodies, consist of nothing but ideas. As he writes in the Introduction to the Inquiry:

as the Bishop [Berkeley] undid the whole material world, this author, upon the same grounds, undoes the world of spirits, and leaves nothing in nature but ideas and impressions, without any subject on which they may be impressed. (Inquiry, Chapter I, Introduction, Section V)
This is the nadir of skepticism. Hume had descended into absurdity through impeccable reasoning from his starting point--Descartes's theory of ideas: "this scepticism is inlaid in it, and reared along with it" (Inquiry, Chapter I, Introduction, Section VII).

Common Sense

Reid thought that the absurd consequences of the theory of ideas are symptomatic of a deeper problem that affected all of modern philosophy. All the modern philosophers treated philosophy itself as autonomous or self-sufficient. More specifically, neither the premises nor the conclusions of the philosophers were sensitive to the principles of common sense. Philosophy chooses its own starting-points, and it decides whether the common-sense principles are justified or not. If they are not, then so much the worse for common sense!

It may be observed, that the defects and blemishes in the received philosophy concerning the mind, which have most exposed it to the contempt and ridicule of sensible men, have chiefly been owing to this--that the votaries of this Philosophy, from a natural prejudice in her favour, have endeavoured to extend her jurisdiction beyond its just limits, and to call to her bar the dictates of Common Sense.(Inquiry, Chapter I, Introduction, Section IV)
Reid held that common sense will always win the contest with philosophy, "for, in reality, Common Sense holds nothing of Philosophy, nor needs her aid" (Inquiry, Chapter I, Introduction, Section IV). Indeed, it is philosophy that stands in need of the aid of common sense to keep it from falling into skepticism.
Philosophy . . . has no other root but the principles of Common Sense; it grows out of them, and draws its nourishment from them. Severed from this root, its honours wither, its sap is dried up, it dies and rots. (Inquiry, Chapter I, Introduction, Section VII)

On the other hand, it is easy to see why philosophers would be inclined to criticize a system that takes common sense as its starting point. The views of ordinary, non-philosophical people seem to be quite unreflective and indeed riddled with error. Philosophers have tended to view their task as being to rise above the common view of things and present a more lofty picture of the universe. Reid does not fall victim to this kind of criticism, however. His account of "common sense," as outlined in the Essays on the Intellectual Powers of Man, Essay VI, Chapter 2, was itself a philosophical one.

Reid begins by explaining what is "common" about "common sense." He thought that common sense is something which nearly all human beings have. The only ones who lack it are defective in some way, victims of "lunacy." A person who, through some natural mental defect, believes he is made of glass has departed from common sense. And a skeptical philosopher who does not believe that mind-independent bodies exist has similarly taken leave of common sense. The difference is that the philosopher abandons common sense not due to a mental defect, but rather due to the influence of metaphysical arguments. So Reid describes a skeptic as a victim of "metaphysical lunacy."

Now what about the "sense" of "common sense?" Here Reid had in mind what we now would call good judgment. When we say of a person that he "has no sense," we describe him as having a strong tendency to misjudge things. On the other hand, a sensible person is one who judges well. Reid regarded this as the fundamental meaning of "sense." He found such a notion in all languages and in all ages.

So nearly everybody has a tendency to make good judgments, at least about everyday matters. This can be seen in the fact that people carry on their day-to-day lives in a more-or-less competent way. But there is more to Reid's notion of common sense than this. Specifically, Reid saw common sense as belief in what is self-evident. We have the beliefs of common sense because God has endowed us with them. Whatever deviates from what is self-evident is, by definition, absurd. So, as Reid saw it, the kind of skepticism he found in Berkeley and Hume is absurd because it contradicts the beliefs of common sense.


It is important to be clear about what Reid meant when he called beliefs of common sense "self-evident." Evidence itself is "whatever is a ground of belief. To believe without evidence is a weakness which every man is concerned to avoid, and which every man wishes to avoid" (Essays on the Intellectual Powers of Man, Essay II, Chapter XX). Beliefs that are self-evident do not depend on anything but themselves as a source of evidence for their truth. Reid follows Aristotle in noting that if there are no self-evident first principles, there would be an infinite regress of reasons. To believe in first principles is as natural as swallowing, and needs as little instruction (Essays on the Intellectual Powers of Man, Essay IV, Chapter IV).

Self-evident beliefs can be divided according to whether they are about what is necessary (what is immutable, and whose contrary is impossible) or about what is contingent (what is mutable, and whose contrary is possible). Beliefs that are self-evident and necessary are called "axioms," and they include basic beliefs about mathematics. Such beliefs are infallible. "The light of truth so fills my mind in these cases, that I can neither conceive nor desire anything more satisfactory" (Essays on the Intellectual Powers of Man, Essay II, Chapter XX). Among the self-evident beliefs about necessary truths are the following:

  • Of grammar: every complete sentence must have a verb.
  • Of logic: no proposition can be both true and false.
  • Of morals: no one ought to be blamed for an action he could not prevent.
  • Of metaphysics:
    • Sensible qualities and thoughts must have a subject.
    • Whatever begins to exist must have a cause which produced it.
    • We can infer with certainty from marks or signs of intelligent design in the effect that its cause designed it intelligently.

What is contingent is the consequence of some will or power. Contingent self-evident beliefs thus admit of the possibility of doubt. In the case of the belief that I see an object before me, for example, "I seem to want [lack] that evidence which I can best comprehend, and which gives perfect satisfaction to an inquisitive mind" (Essays on the Intellectual Powers of Man, Essay II, Chapter XX).

A further distinction can be drawn in the domain of contingent self-evident beliefs. Some are quite general while others are particular. Reid called the former "first principles," including the following (Essays on the Intellectual Powers of Man, Essay VI, Chapter V):

  • The existence of everything of which I am conscious.
  • The thoughts of which I am conscious, are the thoughts of a being which I call myself, my mind, my person.
  • Those things do really exist which we distinctly perceive by our senses, and are what we perceive them to be.
  • The natural faculties, by which we distinguish truth from error, are not fallacious.
  • We have some degree of power over our own actions and what we will.
  • We continue to exist as identical beings, as far back as we can distinctly remember.
  • What is to be in the phenomena of nature will probably be like what has been.
A number of these "first principles" take as self-evident what philosophers from Descartes to Hume struggled to prove on the basis of reason. Particular self-evident beliefs are much less sweeping in scope. An example, made famous by G. E. Moore, might be that there are two human hands in front of me.

Sensation and Preception

In Essay II, "Of Judgment," of Essays on the Intellectual Powers of Man, Reid presents his own theory of perception. We begin with Chapter V, "Of Perception." Perception is an operation of the mind, and as such it is observed directly. But it is only when one has attended to and reflected upon one's perceivings that one gets a "distinct notion" of the operation of perception.

Having attended to and reflected upon his own perceivings, Reid discovers three elements:

  • A conception or notion of the object that is perceived.
  • An irresistible belief that the object presently exists.
  • The fact that the belief in the existence of the object is "immediate, and not the effect of reasoning."
Reid does not try to give an account of how the notion of the object or the belief in the existence of the object is produced through the senses.
If the power of perceiving external objects in certain circumstances, be a part of the original constitution of the human mind, all attempts to account for it will be in vain. No other account can be given of the constitution of things, but the will of Him that made them. (Essays on the Intellectual Powers of Man, Essay II, Chapter V).
He does give an account of the third feature of perception: if our belief were based on reasoning, it would not be present in small children who clearly have it. "The information of the senses is as perfect, and gives as full conviction to the most ignorant as to the most learned."

In addition to the perceiving mind, the operation of perception, and the perceived object, Reid recognizes a fourth element of perception of external objects: sensations. "Almost all our perceptions have corresponding sensations which constantly accompany them" (Essays on the Intellectual Powers of Man, Essay II, Chapter XVI).

Language is such that we often confuse sensations with qualities of perceived objects. Take the expression, "the scent of the rose." It might refer to a sensation "in the mind that feels it" or to a quality in the rose itself. Reid admitted that the sensations we have do not resemble anything in the perceived object, as was recognized by Berkeley.

He had a just notion of sensations, and saw that it was impossible that anything in an insentient being could resemble them; a thing so evident in itself, that it seems wonderful that it should have been so long unknown. (Essays on the Intellectual Powers of the Mind, Essay II, Chapter XVI)
If sensation is identified with perception, then it would follow, as Berkeley understood, that our perception bears no resemblance to a material world, in which case the latter is disposable. But such an identification is an error: Berkeley took "one ingredient of a complex relation for the whole."

If there is no resemblance between our sensations and the objects of perception, then how do we know what qualities the objects of perception have? Reid's answer is found in the first element of perception of an external object: that it always contains a conception or notion of the object perceived. When this notion is "direct and distinct," as it is in the case of "extension, divisibility, figure, motion, solidity, hardness, softeness, and fluidity," then we perceive qualities in the perceived object just as they exist in the object (Essays on the Intellectual Powers of Man, Essay II, Chapter XVII). Thus Reid thought that Locke's notion of primary qualities has "a just foundation" for distinguishing them from secondary qualities.

Nonetheless, Reid saw a problem in Locke's treatment of primary qualities, since Locke claimed that they are known by sensation. Reid comments on Locke's description of the idea of solidity. Even if we could by a difficult abstraction isolate the feeling we get in squeezing a football (as in Locke's example), the feeling "is no more like hardness in a body, than the sensation of sound is like vibration in the sounding body" (Essays on the Intellectual Powers of Man, Essay II, Chapter XVII). The conception of hardness we do have is that of "such a cohesion of its parts as requires a great force to displace them."

The distinctness of our conceptions of primary qualities makes them amenable to scientific investigation. "Their various modifications are precisely defined in the imagination, and thereby capable of being compared, and their relations determined with precision and certainty" (Essays on the Intellectual Powers of Man, Essay II, Chapter XVII).

Our notions of the secondary qualities of the objects of perception are not so distinct as those of primary qualities, and hence "may be a subject of dispute." We do not know what the heat of the fire is, though we know that something in the fire causes our perception of its heat. Thus we have only a relative notion of the quality of heat: it is the cause of known effects. Reid was optimistic that future scientific investigation would reveal the true nature of the secondary qualities.

While sensations do not resemble the bodies that produce them, they nonetheless are the vehicles of our correct conceptions of bodies. Reid does not attempt to give any explanation of the manner in which sensations give rise to the corresponding conceptions.

Consider the sensations produced by my grasping an ivory ball in my hand. The sensation is not round and hard, yet I have a conviction that I am feeling a round, hard body. The connection is simply a design feature of the human being: "By the constitution of my nature, the sensation carries along with it the conception and belief of a round hard body really existing in my hand" (Essays on the Intellectual Powers of Man, Essay VI, Chapter V).

Reid and Hume

A final point concerns Reid's portrayal of Hume as an extreme skeptic. Hume acknowledged that his philosophy is very skeptical, but does it really depart from common sense? Hume explicity recognized that nature prevents us from being skeptics with respect to the independent existence of bodies for more than short periods of time. He conceded that the same kind of judgments Reid called "common sense" come naturally to the ordinary person.

Reid recognizes to some extent that Hume was not a total skeptic.

Indeed, he ingenuously acknowledges, that it was only in solitude and retirement that he could yield any assent to his own philosophy; society, like day-light, dispelled the darkness and fog of scepticism, and made him yield to the dominion of common sense. (Inquiry, Chapter I, Introduction, Section V; see Hume's Treatise, Book I, Part IV, Section 7)
Hume's inability to remain skeptical was taken by Reid to be an inconsistency on his part and a demonstration of the weakness of philosophical reasoning in the face of common sense.

One difference between the philosophers is that Hume described our "natural" beliefs as being the fictitious products of the imagination, rather than as "a gift from Heaven" or a product of "the will of Him who made us." Hume was skeptical about any explanation of the human condition which appeals to the supernatural, especially when a wholly natural explanation can be given in its stead. It is better to recognize the extreme limitations of our power of judgment than to invent a divine basis for judgment which can never be proved to exist or even to be very probable. (See especially his Dialogues Concerning Natural Religion and Section XI of An Enquiry Concerning Human Understanding.)

Reid, on the other hand, held that a belief in intelligent design is self-evident. Moreover, he thought that Hume's attempt to explain all the workings of the human mind by his meager three principles of association produces only a caricature of the human being (Inquiry, Chapter I, Introduction, Section VI). Hume, for his part, believed that with his principles he had properly accounted for all the phenomena of the human mind. This aspect of the debate between Hume and Reid, between a purely empirical account of man and one which situates him in a meaningful, created, universe, continues to the present day.

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