All of Thomas Reid’s thought seems to grow out of his resistance to the skeptical conclusions of Berkeley and Hume. On the one hand, Reid is an effective critic, because he understands their arguments so well. By his own account, he started out as a convinced Berkeleyite and lost his enthusiasm only when he saw the skeptical scorched earth left by Hume. Instead of Berkeley’s real (albeit nonmaterial) and known world, with Hume there is not only no external world, there are no minds, no necessary relations of cause and effect, no rational inductive inferences, etc. Reid decided there must be something wrong, and identifies the problem as the theory of ideas (or “system of ideas” or “ideal system”), which he attributes to Descartes, Malebranche, and Locke (10; information on page references is given at the conclusion of this essay). He argues that Berkeley’s and Hume’s reasoning is mostly correct, derived by pursuing the theory of ideas more consistently than Descartes, Malebranche, and Locke had done. Thus, the extreme conclusions of Berkeley and Hume were due not to errors of reasoning but to errors of their starting points.

On the other hand, Reid’s effectiveness is undercut, in my view, by his failure to supply a convincing alternative to the theory of ideas. He should not be blamed for this. After all, it took philosophy more than 200 years after Reid to resolve the errors of the theory of ideas and put the philosophy of perception on a new footing. In my view, this has now been done with Intentionalism, and we shall see that Reid made promising steps in this direction. But he doesn’t go far enough, and as a result, he does not produce a fully convincing account of perception. He also fails to produce a plausible account of the contribution of the senses to conceptual thought, as the theory of ideas claims to do. Finally, I think Reid’s appeal to “common sense” is another reason he failed to convince. The notion of the authority of common sense—as a set of supposedly unchallengeable, quasi-axiomatic tenets, such as that there is an external world that we know by sense-perception—is fundamentally dogmatic and anti-intellectual, and it’s no surprise that philosophers generally denigrate it.

Thus, although Reid produced trenchant criticisms of the theory of ideas, these did not get the attention they should have. Every theory has problems and inadequacies. Accordingly, thinkers do not abandon a theory merely because it is in difficulties and arguably falsified or refuted. Rather, people abandon a theory when they have a better theory. Now that this is in our grasp, Reid suddenly looks a lot smarter than he used to.

In what follows, I explain Reid’s account of perception and show how it addresses certain failings of the theory of ideas in a way that fits comfortably with present-day developments in the philosophy of perception. I conclude by pointing out some problems with Reid’s theory.

The Theory of Ideas

As deployed by Berkeley and Hume, the theory of ideas says that our original mental contents are perceptions or sense-impressions. For example, I see a tree in my yard, a book on my desk, etc., and there is formed in my mind a visual image of a tree, a book, etc. They call these images, and all mental contents generically, of whatever sort, ideas. It should be stressed that these “ideas” are mental objects. They are in fact the exclusive objects of human knowledge. (See Berkeley, Principles, §1; Locke, Essay, “Introduction,” §8.) Thus, when I see a tree in my yard, the immediate object of my awareness is a mental object, an idea of a tree. My awareness of the physical tree—if indeed I have any such awareness, which Berkeley and Hume deny—is only mediate or indirect.

The notion that the immediate objects of sense-perception are ideas was accepted by nearly all philosophers, with only minor variations, until at least the mid-20^th century. (And even longer if we discount certain late attempts to fundamentally avoid it—in my view unpersuasive—some of them amusing [J.L. Austin], some not [Heidegger].) It eventually became common to replace “idea” with “sense-datum,” and in the 20^th century, the theory of ideas became known as the sense-data theory (at least for sense-perception).

Now, all other mental contents are derived from these sensory “ideas” (and mental activities—but leave these aside). This is the distinctive claim of British empiricism, expressed in Russell’s dictum that, “Every proposition which we can understand must be composed wholly of constituents with which we are acquainted.” (The Problems of Philosophy, 58.) Of course, the 17^th and 18^th century philosophers didn’t have Russell’s theory of descriptions; nevertheless, what he is articulating is a descendent of their doctrine. For Berkeley and Hume, non-perceptual mental contents consist of fainter, less vivid and forceful copies of sensory ideas. Thus, my recollection of a tree will be a fainter copy of a sensory idea of that tree. My imagining of a tree will be constructed out of a memory of a tree or pieced together from multiple copies or parts of copies. My general concept of a tree might be an exemplary image of a tree that is thought to cover any objects sufficiently similar to it. And so forth. This is how the theory of ideas accounts for the contribution of the senses to the contents of conceptual thought.

Reid’s View

Reid rejects the theory of ideas in its entirety. It is all bunk. There are no “ideas.” According to Reid, the phenomenological elements of perceptions are merely sensations, which serve as signs of their external objects. Just as pain, for example a burned finger, is a sensation that signals some damage to your body, so all sensible qualities are perceived by sensations that signify objective properties of their objects. Suppose you hold an ordinary wooden pencil in your hand. You press it between your fingertips and feel its hardness. That is, you experience a sensation in your fingertips, and as a result you conceive the hardness of the pencil. Hardness, according to Reid, is nothing like the sensation by which you perceive it (Inquiry, ch. 5, sec. 2). The sensation is a certain feeling; hardness is the firm cohesion of a thing’s parts as a result of which it does not easily change its shape. The feeling and the cohesion are as unlike as chalk and cheese. Therefore, no amount of reasoning will allow you to discover the hardness of the pencil from the sensations it excites in you. Instead, the conception of hardness is naturally prompted by the sensation. That is, you are innately constituted to conceive the hardness of the pencil in your hand when you feel the appropriate sensation.

All sense-perception works like this. The pencil may also have a temperature that you feel with your fingers. For instance, it might feel warm if it was recently sitting on the radiator. It has a spatial extension that you can feel with your fingers and also see. It has a color that you see, and so forth. In every case, the sensation is nothing like the property it signifies. There is no necessary or intrinsic relation between the sensation and the property—any more than between the linguistic sign “gold” and the metal—and no amount of reasoning will lead you from the first to the second. Rather, the sensation is a natural sign of the property. Sometimes, as with hardness and extension, our senses give us a clear and distinct conception of the property (51–52). In other cases, as with hot and cold and color, our senses give us a conception only of some I-know-not-what property in the object as being the cause of the respective sensations. That is, although we know precisely what hardness and extension are (and figure and space and motion), properties like being hot or yellow are unknown. Yellow is that property of an object by which it causes us to have a yellow sensation when we look at it. It is an objective property of a body, but our senses do not enable us to know exactly what property. This marks the distinction, for Reid, between primary and secondary qualities: the primary qualities are those for which our senses give us clear and distinct conceptions, and the secondary qualities are those for which they don’t.

Thus, for Reid, perception is conceptual. The model is:

perception = sensation + conception

The sensation in itself conveys no information. It is an arbitrary sign. There is no reason whatsoever, as far as Reid can see, why hardness should not have been perceived by the sensation by which in fact we perceive yellow and yellow should not have been perceived by the sensation by which in fact we perceive hardness. However, it is a fact of our innate constitution that these sensations convey to us the conceptions they do. In this way, sensations are “natural” signs. As a result, our perception of the respective properties of bodies is mediated by these sensations.

Finally, besides a conception of the object and its properties, the perception of an external body also produces in us “an irresistible conviction and belief of its present existence” (160). And this of course is no more the product of reasoning than is the conception of the object and its properties.

All this is nearly the reverse of the theory of ideas. For, the “ideas” of this theory are just what Reid is calling sensations. And recall that for the theory of ideas, “ideas” are the objects of perception. At least, they are the immediate objects. This is just why philosophers such as Descartes, Malebranche, and Locke—and later, Moore and Russell—faced the problem of showing that sense-perception yields any knowledge of external objects. And in the “phenomenalist” limit of this theory, expressed by the main line of philosophy running from Berkeley, Hume, and Mill through Ayer and C. I. Lewis, ideas are the sole objects of perception. In this view, there are no further objects. The world is a construction of sensations. Recall Mill: matter is “a permanent possibility of sensation.” (Examination of William Hamilton’s Philosophy, ch. 2. N.B.: Hamilton was Reid’s editor and acolyte.)

So, for the theory of ideas, sensations are the content of perception, and the notion that the types of sensations associated with different perceptions might be interchanged without a change of what is perceived is bizarre and impossible. On this theory, the world of our experience just is a world of sensations, whereas for Reid, sensations are merely signs of external things, signs to which we normally pay little attention, just as in natural language we normally take little notice of the words themselves but concentrate our attention on the things they signify (38–39). For Reid, not only is there no internal or necessary relation between sensations and the things they signify, but sensations are not even always necessary for perception. In an interesting passage on the visual perception of figure, he argues that we need have no visual sensations at all; the physical effect of light rays on our retinas may be sufficient to suggest to us the position and shape of an object with no intermediate conscious sign (75–76).

But it is not just that, for Reid, perception is conceptual. The really important difference between Reid and his opponents is that, for him, perception is intentional. That is, perceptions have intentionality: they are about objects other than themselves. This is a crucial property of perception and of thought generally, and “ideas” do not have it. This is most apparent in the phenomenalist variant of the theory of ideas, where sensations are the only objects of perception and the world is a construction of sensations. Here nothing else exists for sensations to be about. But for the representationalist (indirect realist; i.e., Locke et al.) variant as well, the sensations are not inherently about distal objects, which must be inferred from or associated with sensations by some further mental process. Indeed, this problem infests the whole theory of ideas, including its account of higher cognition. For example, a memory of an event is supposed to be a copy that is less forceful and vivid than the original perception. But what makes the copy be about its original? The copy is a causal effect of the original. But so what? Smoke is a causal effect of fire, and broken glass is a causal effect of being smashed by an errant baseball, but smoke is not about fire and broken glass is not about baseball smashings. Aboutness is a different sort of animal from causation. We might infer a cause from its effect. But that is a separate process. And how will we do that? To infer anything requires that we be able to form propositions—thoughts that have intentional content (i.e., that are about something other than themselves). And the truth is that the theory of ideas makes no provision for our ability to do this. Proponents of the theory of ideas take for granted that many of our thought processes are in fact intentional, but without making intentionality any part of their theory or specifying which processes have intentionality and which do not.

The failure to take account of intentionality is a problem because it prevents the theory of ideas from being able to say that any mental states are states of awareness of anything beyond themselves. It thus lies at the root of most of Berkeley’s arguments against the conceivability of matter and any external world. Indeed, it lies at the root of the theory of ideas itself. Fundamentally, it is only if we cannot say that a perception is a state of awareness of a distal object that it makes sense to say that the object of a perceptual state is a sensation—which is really a way of saying that a perceptual state is its own object.

As an illustration of these claims, consider the following famous passage from Hume:

We may observe, that it is universally allowed by philosophers, and is besides pretty obvious of itself, that nothing is ever really present with the mind but its perceptions or impressions and ideas, and that external objects become known to us only by those perceptions they occasion. …

Now since nothing is ever present to the mind but perceptions, and since all ideas are derived from something antecedently present to the mind; it follows, that ‘tis impossible for us so much as to conceive or form an idea of any thing specifically different from ideas and impressions. Let us fix our attention out of ourselves as much as possible: Let us chase our imagination to the heavens, or to the utmost limits of the universe; we never really advance a step beyond ourselves, nor can conceive any kind of existence, but those perceptions, which have appeared in that narrow compass.

(Treatise, I.ii.vi)

There are two striking points here. First, “nothing is ever really present with the mind but its perceptions or impressions and ideas.” This is to say what I put just now by saying that, in the theory of ideas, “a perceptual state is its own object.” (And on the theory of ideas, this goes not only for sense-perceptions, but for all mental states.) Perceptual states do not have intentionality: they are not states of awareness of something other than themselves. This is the premise. The second point is that from this premise, mischief follows: “it follows, that ‘tis impossible for us so much as to conceive or form an idea of any thing specifically different from ideas and impressions.” Since the objects of perception are “ideas”—sensations—not external objects, we have acquaintance only with sensations, not with external objects. Now, external objects—material bodies—are supposed to be something radically unlike (“specifically different from”) sensations. And how can we imagine something radically unlike anything we have ever experienced? It would be like a person who was blind from birth trying to imagine color. Hence, we can form no idea of material bodies: “we never really advance beyond ourselves.”

Reid is right to identify the theory of ideas as the root cause of skeptical conclusions of this sort. And the root problem with the theory of ideas is that it doesn’t provide for the intentionality of our mental processes. And this problem is irreparable: absence of intentionality lies at the heart of the theory of ideas. The theory of ideas with intentionality added would simply no longer be the theory of ideas.

Hence, in my view, the most important and distinctive feature of his theory of perception—practically unique among early modern philosophers, as far as I know—is its explicit intentionalism. Strictly speaking, it is his theory of conception that is explicitly intentionalist. (See Essays on the Intellectual Powers, Essay IV.) However, sense-perception is intentional because, as we have seen, for Reid, perception is conceptual. Reid’s theory of conception is intentionalist because it holds that (a) the immediate object of a conception is remote from the conceptual state, (b) there is no mental image or “idea” or any other intermediate object, (c) the mental process of conceiving and the object conceived are two different things that must be sharply distinguished, and (d) the object of a conception might not exist (225–226). Those who are familiar with intentionalism in the philosophy of mind will recognize that this is an intentionalist theory. This is not the place for a discourse on intentionalism, but let me make two brief remarks on its significance for the theory of ideas.

First, intentionalism about mental states holds that they can be about some object or content immediately, without any further, mediating representation. It is in this way analogous to the theory of “direct reference” in the philosophy of language. How the intentionality of a mental state arises is not further explained, as a rule. Rather, it is taken for given that some mental states simply have the power to be about objects other than themselves. If this seems like dodging the problem of explaining aboutness, we should remember that to explain aboutness is not necessarily our job. (We should also remember that the theory of ideas does not attempt to explain how we have the power to experience “ideas,” but this was not regarded as a problem for that theory.) Moreover, we evidently do not know how to explain it. The theory of ideas, insofar as it was an attempt to explain aboutness, was a colossal failure. Meanwhile, it seems undeniable that some mental states represent their objects immediately. If we can build a satisfactory theory of perception on this basis, there seems to be no reason why we shouldn’t. The payoff for a theory of sense-perception is direct realism. On an intentionalist theory of perception, if I see a tree in front of me, my visual state of seeing the tree has the tree as its immediate object. There is no mediating “idea” or sense-datum of the tree.

Second, intentionalism holds that an intentional state can be about an object even if that object is fictional or otherwise does not exist. Intentionality is the power of a mental state to represent a remote object; it does not depend on the existence of that object. Thus, I can think about the coffee cup on the desk in front of me; the bust of George Washington on Mount Rushmore thousands of miles away from me; George Washington himself, who longer exists; and Bilbo Baggins, who never existed. This is a puzzling feature of intentionality, but again it is normally taken for given that our minds have the power to do this, even if we do not understand exactly how. The payoff here is to provide an alternative account of illusion and hallucination. The theory of ideas explains these by positing “ideas.” For example, when a straight stick plunged halfway into a pool of water looks bent at the waterline, we are visually presented with a bend that is not the bend of the physical stick. What, then, is bent? The theory of ideas answers that a mental object—the sense-datum of a stick—is bent. Indeed, this is offered as a prime reason to believe in sense-data: sense-data explain how we are able to experience illusions and hallucinations by providing mental objects to be the bearers of content that has no physical reality. But according to intentionalism sense-data are not required for this. Instead, we can merely note that intentional perceptual states have the power to represent contents, like bends in sticks, that do not exist.

Finally, consider the passage from Hume’s Treatise quoted above. Hume says that nothing is ever present to the mind but perceptions—i.e., interior objects, such as sense-data; accordingly, we have no means of conceiving of external objects. Intentionalism replies that this fails to keep clear the distinction between the state of perceiving and the object perceived. Thus, when I see a tree in front of me, what is present to my mind is the tree, an external object. Of course, I see it by means of a visual state, which is internal. But the visual state is not the object. By using “perception” to refer to both, Hume fails to keep the distinction clear and falls into confusion. There is a similar ambiguity in Hume’s use of “present to the mind,” which can mean alternately within the mind and object of awareness. “Idea” itself, as employed in the theory of ideas, is ambiguous in the same way: it means alternately mental state and mental object. It is a virtue of intentionalism that it lays stress on keeping these distinctions clear. Thomas Reid’s account of perception benefits from all these features of intentionalism and in this way provides an account of perception markedly superior to the theory of ideas.

Problems with Reid

If Reid’s theory of perception is so superior, why didn’t it take the world by storm? Three reasons strike me as especially noteworthy. The first and most important is Reid’s failure to extend his intentionalism to the sensory content of perception. As we have seen, Reid treats this content as sensations—i.e., as sensory accompaniments of perception that our minds are innately constituted to take as signs of objective properties. For example, there is a sensation of touch by which we are led to conceive the hardness of an external object, another by which we are led to conceive its extension, and still others for texture, size, shape, weight, and so on. And these sensations in themselves reveal nothing about the properties they lead us to conceive, but instead are arbitrary signs of their properties, just as “gold” is an arbitrary sign of a certain metal. Now, all of this is surely false. Take Reid’s own example of hardness. It seems evident that the “sensation” one feels when pressing an ordinary pencil between one’s fingertips is that of its not yielding. Which is just its being hard. Now, this is representational content. That is, the “sensation” is not of nothing, nor is it unrelated to the property of hardness. Indeed, it seems to be a feeling of hardness. Reid’s attempt to deny this is based on identifying hardness as “the firm cohesion of the parts of a body” (39), which indeed does not seem to be the same thing as the way hardness feels. But a “firm cohesion of the parts of a body” is a particular explanation of hardness that need not be universally true. For example, a bladder inflated to an enormous pressure is hard. What is universally true, it seems to me, is just that the surface is unyielding, for whatever reason. That is just what you feel by touch; that is, it is information conveyed in the sensation.

For a different example, think of the visual appearance of a square. What shape is it? Square! If you exchanged it for the visual sensation of yellow or a circle, would it still make you understand a square shape? No! Indeed, the suggestion is bizarre. Hence, sensory states have representational content. They are not just signs that trigger conceptions to which they have no internal or necessary relation. Indeed, this fact is what gives the theory of ideas much of its plausibility. Berkeley’s suggestion that we live in a nonmaterial world of sensory “ideas”; Hume’s suggestion that we “always suppose the very images, presented by the senses, to be the external objects, and never entertain any suspicion, that the one are nothing but representations of the other” (first Enquiry, Sec. XII, Part I); the suggestion that conceptions and indeed all contents of higher cognition consist of less vivid and forceful copies of sensory images; all these testify to the fact that sensory images (visible, tactile, etc.) themselves represent whatever properties or objects they are about.

Thus, Reid’s attempt to deny this is a serious failing of his theory of perception. (He claims that philosophers have only believed it because sensations as such have no names and are hardly noticed as the mind passes quickly to the conceptions they trigger. In sense-perception, “the mind immediately passes to the thing signified, without making the least reflection on the sign, or observing that there was any such thing” (47–48). That’s right, the claim is that we fail to observe that there is any sensory part of sense-perception!) What Reid should have done is extend his intentionalist account of conception to sensation. Sensory images, whether visual, tactile, auditory, or otherwise, are mental states that are about something other than themselves, just as concepts and thoughts are. This doctrine does away with the theory of ideas as well as Reid’s own scheme, without needing his doctrine of conceptions. Just this program has in fact been widely adopted in contemporary philosophy of perception over the past three decades. (For one example, see Michael Tye, Consciousness, Color, and Content, 2000.)

This approach also enables us to explain the role of sense-perception in supplying content for higher cognition. It seems likely that concepts are formed somehow from percepts, and—whether they are or not—that a significant element of conceptual content is derived from percepts. Reid’s theory must deny this, since it holds that content originates with concepts. This is a second serious failing of his theory. It would be avoided by adopting an intentionalist account of sensory images.

Finally, a third problem for Reid, which may have contributed to his being relatively neglected, is his marching under the banner of “common sense.” Common sense in philosophy, for Reid as well as for G.E. Moore—the other main exponent of common sense in the history of philosophy—seems to amount to the following. There are certain principles or tenets that should be treated as foundational, basically because they are inescapable. Some examples: that one’s thoughts and sensations exist (267–268); that there is a being, oneself, that is the subject of one’s thoughts and sensations (58; 269); that the things one distinctly remembers really happened (270); that the objects and properties we perceive by our senses really exist (58; 272); that the course of nature is steady and uniform (58); and that the natural faculties by which we distinguish truth from error are not fallacious (275). (A long list is provided in Intellectual Powers, Essay Six, Ch. 5.) These tenets have a status analogous to that of axioms in geometry (58). Namely, they are the starting points of reason. Therefore, there can be no reasoning concerning their truth. Nor can they be subject to rational questioning. The ultimate justification for them, if any, is that we are compelled to accept them. They are baked into our constitution, and we cannot help but believe them (57–58).

The trouble with this approach is that the tenets in question do not in fact have an axiomatic status that would put them beyond the scrutiny of reason. Bizarre as it may seem to suppose that I have no self or that my memories are all false or that there are no external objects, these suppositions do not make reasoning impossible. Some of them, such as that the course of nature is steady and uniform and that our cognitive faculties are intact, would, if false, make discovery very hard, but that is not the same as making reasoning impossible. Or, if these particular problems should turn out to be so severe that reasoning literally could get nowhere, then we could not be having this or any discussion. In other words, these tenets are prerequisites for our knowledge of the world to grow in the way we are accustomed to think normal, but not for knowledge per se (given that we can pose questions about them). Of course, there are some genuinely axiomatic principles of reason, which therefore cannot be proved by reason. The law of non-contradiction comes to mind. Interestingly, it is not on Reid’s list.

Thus, the justification for the tenets of common sense comes down to the claim that we cannot help believing them (57). To his credit, this is Reid’s most common defense of them. That is, he does not claim that they are evident, only that they are compulsory. Of course, just because we cannot help believing something does not make it true. However, it does not even seem to be true that we cannot help believing them. When Neo is confronted with strong enough evidence that he is in the Matrix and his phenomenal life is not real, he believes it, and this does not seem unrealistic. If the phenomena of nonlocality in quantum mechanics can cause us to seriously doubt whether the properties of electrons are local to the electrons, then we can seriously doubt any of the tenets on Reid’s list.

In the end, the appeal to common sense seems dogmatic. It is an attempt to place certain tenets beyond question. The tenets in question are admittedly fundamental to our ordinary conception of the world—close to the center of Quine’s web of belief. All the more reason, then, why we should be able to rationally justify them. It should not be surprising if philosophers are turned off by “common sense.” Philosophy is about raising fundamental questions and attempting to answer them. Those whose way with such questions is to declare them off-limits may be in the wrong line of work.

Reid Citations

Reid wrote three major works:

• An Enquiry into the Human Mind on the Principles of Common Sense, 1764.
• Essays on the Intellectual Powers of Man, 1785.
• Essays on the Active Powers of Man, 1788.

Page references in this post are to Thomas Reid, Inquiry and Essays, an abridgment edited by Keith Lehrer and Ronald E. Beanblossom, Bobbs-Merrill, 1975.