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| Thomas Reid |
Thomas Reid was the founder of the Scottish "Common Sense" school of philosophy. A contemporary and critic of David Hume, he is best known for his staunch defense of common sense and trenchant opposition to the "way of ideas," the theory that the immediate objects of perception and other cognitive acts are always internal images or ideas, not external physical objects.
His views exerted a good deal of influence until the mid-nineteenth century or so, when they began to be eclipsed by absolute idealism, pragmatism, and other philosophical movements, but they have been the subject of renewed interest from the 1970s on.
After being educated at Marischal College in Aberdeen, Scotland, Reid served for fifteen years as a parish minister in nearby New Machar. In 1752 he was appointed professor at King's College in Aberdeen, where he taught mathematics, physics, and philosophy, among other subjects.
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| Marischal College |
He tells us that in his youth he believed nearly the entire philosophy of George Berkeley but that a reading of Hume's Treatise of Human Nature (1739–1740) convinced him (by carrying Berkeley's philosophy to its logical conclusions) that there must be some original defect in it.
Reid identified this defect as the theory of ideas, which he went on to challenge in college lectures, meetings of the Aberdeen Philosophical Society, and two books. In 1764 he published his first major work, An Inquiry into the Human Mind on the Principles of Common Sense, in which he set forth his reasons for opposing the theory of ideas and offered an alternative theory of how we gain knowledge by means of the various senses.
In the same year he accepted the chair in moral philosophy at Glasgow, succeeding Adam Smith. He lectured there until 1780, when he resigned to prepare his last two major works: Essays on the Intellectual Powers of Man (1785), devoted to the contributions of perception, memory, reason, and other cognitive powers to human knowledge, and Essays on the Active Powers of the Human Mind (1788), devoted to the nature of action, will, freedom, and morality.
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| Adam Smith |
This article provides a synopsis of Reid's main views more or less in the order in which he presented them in his three published books: the Inquiry (abbreviated as Inq.), the Intellectual Powers (abbreviated as IP), and the Active Powers (abbreviated as AP). Numbers separated by a period refer to chapter and section numbers in the Inquiry and to essay and chapter numbers in the two volumes of Essays.
Critique of the Theory of Ideas
Almost alone among the great modern philosophers, Reid espoused a direct realist theory of perception. He repudiated the assumption that what is immediately present to the mind is never an external thing, but only an internal image, impression, representation, or (to use the most common eighteenth-century term) idea.
Ideas were usually conceived of as mental entities that existed only as long as the mind was aware of them. Reid found the theory of ideas to be taken for granted in the work of most of his philosophical predecessors, including René Descartes, Nicolas Malebranche, Antoine Arnauld, John Locke, Berkeley, and Hume
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| Nicolas Malebranche |
Some of these philosophers (for example, Descartes and Locke) were realists, believing that ideas are caused in us by physical objects existing outside the mind. Others (notably, Berkeley) were idealists, repudiating the existence of a world outside the mind and believing that the things we call physical objects are simply bundles of ideas.
In either case, we are cut off from direct perception of the physical world, either because there is no physical world to be perceived, or because our perception of it is indirect—not strictly perception at all, but inference based on what we do perceive, namely, ideas.
Reid makes at least three important points against the theory of ideas. First, the arguments in favor of the theory are weak and without cogency; second, the theory does nothing to explain how perception is possible; third, the theory stands in the way of our knowing or even being able to conceive of the physical world.
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| perception is possible |
One of the arguments for the theory of ideas that Reid singles out for criticism is a version of the argument from perceptual relativity. Hume had claimed that the "universal and primary opinion of all men" that they perceive external objects directly is "destroyed by the slightest philosophy," offering the following argument in section 12 of the Enquiry concerning Human Understanding: "The table, which we see, seems to diminish as we remove further 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."
Hume's argument may be cast into the following syllogism:
- What I see diminishes in magnitude as I retreat from it;
- The table itself does not diminish in magnitude as I retreat from it;
- Therefore, what I see is not the table itself (but only an image or idea).
- What I see diminishes in apparent magnitude as I retreat from it;
- The table itself does not diminish in real magnitude as I retreat from it;
- Therefore, what I see is not the table (but only an image or idea).
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| syllogism |
The real magnitude of an object (for example, the edge of a table) is an intrinsic property of it, measured in feet or inches, whereas the apparent magnitude of an object is a relation between the object and a perceiver (or his vantage point), measured by the angle the object subtends at the eye. Reid takes the terminology of "real" versus "apparent" from the astronomy of his day; it is not necessarily implied that there is anything illusory about apparent magnitude.
It is easy to see that apparent magnitude varies with the distance between object and perceiver (objects subtending smaller angles when further away) whereas real magnitude does not. Once we record these facts correctly, as in Reid's version of the syllogism, we see that the conclusion of the argument does not follow from the premises.
Moreover, Reid would resist the thought that if O has greater apparent magnitude when seen from p than when seen from p', that is because it presents a larger image to the observer at p than to the observer at p'. Apparent magnitude is a strictly dyadic relation, involving only the object and the perceiver (or his vantage point) and no third thing such as a mental image.
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| mental image |
Reid's second point against the hypothesis of ideas is "that ideas do not make any of the operations of the mind to be better understood". Ideas had been thought necessary to explain how we perceive things that are distant, remember things that are past, or imagine things that do not exist at all, but Reid argues that all such explanations are worthless. They presuppose that ideas themselves can somehow be of the remote, the past, or the nonexistent.
Sensation and Perception
A sensation is an event that occurs in a sentient subject when he or she smells a rose or tastes a fig. It lacks figure and extension and other qualities of bodies, being entirely mental. Reid calls sensations "principles of belief," by which he means that when we have a sensation and attend to it, we cannot help believing that it exists, that a subject of it exists (ourselves), and that some external object (for example, some quality in the rose) exists as its cause.
Some critics of Reid have thought that his sensations are simply ideas under a new name, but there are important differences—especially if he holds an adverbial theory rather than a theory that divides sensation into act and object. If sensing required its own special objects, the argument from perceptual relativity against direct realism could be reinstated.
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| special objects |
The mountain that looks blue from a distance and green from close up would do so by generating first blue and then green sensory objects in my mind, and these variously colored objects would have to be distinct from the unchanging mountain. They would displace the mountain itself as my object of direct awareness. But Reidian sensations do not have objects to get in the way of direct perception of external things.
Although sensations do not have objects, they can become objects for us, in the sense that we can know through proper attention what sorts of sensations we are having. Indeed, Reid thinks that if we attend carefully to our sensations, we can know perfectly what they are like and can scarcely make any mistake about them.
Yet typically we pay so little attention to them that we become almost oblivious to them; they serve as mere cues or signs from which our minds leap instantly to other things that they signify.
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| conception and belief |
Although Reid says that sensation generally serves as the trigger for the conception and belief involved in perception, perception proper is just conception plus immediate belief.
Reid thinks it possible that perception should occur in the absence of sensation, and he holds that there is one variety of human perception that actually does occur without any characteristic sensation: namely, the perception of visible figure. Reid thus deemphasizes the role of sensation in perception in a way that some contemporary theorists (for example, James J. Gibson) would applaud.
By the same token, however, his threefold definition may strike others as leaving out precisely that by which a genuine perception of a snake in the path ahead is distinguished from the conception and immediate belief in it one may form as the result of a friend's warning.
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| Bertrand Russell |
Here Reid's views may gain in plausibility if we reckon his "conception" as something like what Bertrand Russell called knowledge by acquaintance. It is not necessarily the exercise of a concept in mere thought.
Reid's Nativism
Reid thought that much of what he found alarming in Hume's philosophy stemmed from Hume's adherence to the empiricist maxim that we have no ideas or notions that are not derived from previous impressions or sensations.
It is by this principle that Hume was led to conclude that we have no legitimate notions of objects existing unperceived, of causal connections amounting to more than constant conjunctions, and of a self that is the subject of various mental operations.
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| mental operations |
Reid sought to overthrow Hume's philosophy by undermining its foundations, and for this purpose he tackled the empiricist principle head-on. He pointed to a notion that he thought Hume would surely concede that we possess— the notion of extension, or being spread out in space— and contended that this notion lacks a proper Humean birthright in our sensations.
If it were once acknowledged that not even so uncontroversial a notion as extension can be extracted from our sensations or impressions, Reid thought, the way would be open for recognizing the legitimacy of other notions with no sensory origin, such as the ideas of agency, self, and an external world.
To back up his contention that the notion of extension is not derived from sensation, Reid offers a thought experiment he calls his experimentum crucis. He asks us to imagine a being furnished with a progressively richer array of sensations, beginning with those caused by the prick of a pin, advancing to more complex sensations such as those caused by the pressure of a blunt object against his or her body, and culminating with the sensations accompanying the motion of his or her limbs. He asks at each step in the series whether those sensory materials would suffice to give a being who reflected upon them a conception of extension, and his answer is no.
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| experimentum crucis |
Positively, Reid's doctrine is that the conception of extension is innate—not in the sense that we have it from birth, but in the sense that it is triggered in us by certain sensations from which it could never have been derived from any process of abstraction or ratiocination. We are enabled to form the conception of extended things only because we are innately programmed to do so.
For further light on the import of Reid's nativism, we may restate it in terms of the threefold classification of natural signs he offers in sections 4.2 and 5.3 of the Inquiry. Reid first divides signs into the artificial and the natural.
In the former class, the connection between sign and thing signified is established by compact or convention, as with the words of human language. In the latter class, the connection between sign and thing signified is established by nature, as with smoke and fire and other cases of effect and cause. Reid then further divides natural signs into three classes.
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| three classes |
In the first class, the connection between sign and thing signified is "established by nature, but discovered only by experience", as in the example of smoke and fire already given. In the second class, the connection is "not only established by nature, but discovered to us by a natural principle, without reasoning or experience".
Reid thinks that certain features of the human countenance are signs in this sense of thoughts and other mental states. For example, an infant is innately disposed to read a smile on its mother's face as a sign of approval without having to learn this connection through experience.
Unless there were a basic repertoire of natural signs of this second class, Reid believes, the signification of artificial signs could never be agreed upon or learned. Finally, in the third class are those signs "which, though we never before had any notion or conception of the thing signified, do suggest it, or conjure it up, as it were, by a natural kind of magic".
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| natural kind of magic |
Not only is the connection between sign and thing signified innately programmed into our constitution (as with signs of the second class), but also the very notion of the thing signified is innate in the sense that it is in no way derivable by abstraction from any of our sensations. Reid believes that the tactile sensations to which we respond with conceptions of extended bodies are natural signs belonging to this third class.
Reid takes his nativism to afford an answer to an argument for skepticism he finds embodied in the combined philosophies of Berkeley and Hume. He formulates the argument as follows (Inq 5.8, p. 75): (1) We can have no conception of anything but what either resembles or is deducible from our sensations; (2) nothing resembles or is deducible from sensations but other sensations; (3) therefore, we can have no conception of anything but sensations.
If the argument were correct in both its premises, it would follow that we cannot even conceive of, let alone have knowledge of, a world lying beyond our sensations. Reid thinks the second premise is correct, and he credits Berkeley with having made it evident.
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| beyond our sensations |
But he thinks the first premise—which states in Reid's language Hume's principle that all our ideas are copied from precedent impressions—is false. "That we have clear and distinct conceptions of extension, figure, motion, and other attributes of body, which are neither sensations, nor like any sensation, is a fact of which we may be as certain, as that we have sensations".
The Mechanics and Geometry of Vision
More than half of the Inquiry is devoted to vision, which Reid regards as the noblest of the senses. It informs us of the properties of objects far distant, such as the sun and the moon, and it can disclose in a glance the figure of a cathedral, whose delineation by touch would be the work of a lifetime.
Reid provided solutions to a number of puzzles about vision that lie today within the province of cognitive science rather than philosophy. For example, why do we see things upright despite having inverted retinal images of them? To explain this, Reid appeals to the law that an object will be seen in the direction of a straight line drawn from the point of retinal stimulation through the center of the eye and into ambient space.
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| retinal stimulation |
Why do we normally see objects single despite having two retinal images of them, yet under certain circumstances see them double? Reid's answer appeals to the law of corresponding retinal points: If rays from an object fall on points of the two retinas lying at equal distances and directions from their centers, the object will be seen as single, but otherwise as double.
One of Reid's more remarkable findings is that the visible figures of objects are governed by a non-Euclidean geometry. Reid believed that sight by itself (before we have learned any correlations with touch) informs us only of the two-dimensional spatial features of objects. Although the objects we see are at a distance from us (pace Berkeley), the eye is incapable of making any discriminations of depth.
To an eye placed at the center of a sphere and looking out, great circles on the surface of the sphere (whose outward curvature is invisible to the eye) must appear as straight lines, and every figure seen by the eye must have the same geometrical properties as some figure drawn upon the sphere.
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| geometrical properties |
In consequence of this, Reid argued that the geometry of visibles is what we would nowadays classify as a Riemannian geometry. A visible triangle, unlike a triangle perceived by touch, always has an angle sum at least slightly greater than 180 degrees, and no two visibly straight lines are ever strictly parallel
