Thomas (1710–96), Scottish philosopher, a defender of common sense and critic of the theory of impressions and ideas articulated by Hume. Reid was born exactly one year before Hume, in Strachan, Scotland. A bright lad, he went to Marischal College in Aberdeen at the age of twelve, studying there with Thomas Blackwell and George Turnbull. The latter apparently had great influence on Reid. Turnbull contended that knowledge of the facts of sense and introspection may not be overturned by reasoning and that volition is the only active power known from experience. Turnbull defended common sense under the cloak of Berkeley. Reid threw off that cloak with considerable panache, but he took over the defense of common sense from Turnbull. Reid moved to a position of regent and lecturer at King’s College in Aberdeen in 1751. There he formed, with John Gregory, the Aberdeen Philosophical Society, which met fortnightly, often to discuss Hume. Reid published his Inquiry into the Human Mind on the Principles of Common Sense in 1764, and, in the same year, succeeded Adam Smith in the chair of moral philosophy at Old College in Glasgow. After 1780 he no longer lectured but devoted himself to his later works, Essays on the Intellectual Powers (1785) and Essays on the Active Powers (1788). He was highly influential in Scotland and on the Continent in the eighteenth century and, from time to time, in England and the United States thereafter. Reid thought that one of his major contributions was the refutation of Hume’s theory of impressions and ideas. Reid probably was convinced in his teens of the truth of Berkeley’s doctrine that what the mind is immediately aware of is always some idea, but his later study of Hume’s Treatise convinced him that, contrary to Berkeley, it was impossible to reconcile this doctrine, the theory of ideas, with common sense. Hume had rigorously developed the theory, Reid said, and drew forth the conclusions. These, Reid averred, were absurd. They included the denial of our knowledge of body and mind, and, even more strikingly, of our conceptions of these things.
The reason Reid thought that Hume’s theory of ideas led to these conclusions was that for Hume, ideas were faded impressions of sense, hence, sensations. No sensation is like a quality of a material thing, let alone like the object that has the quality. Consider movement. Movement is a quality of an object wherein the object changes from one place to another, but the visual sensation that arises in us is not the change of place of an object, it is an activity of mind. No two things could, in fact, be more unalike. If what is before the mind is always some sensation, whether vivacious or faded, we should never obtain the conception of something other than a sensation. Hence, we could never even conceive of material objects and their qualities. Even worse, we could not conceive of our own minds, for they are not sensations either, and only sensations are immediately before the mind, according to the theory of ideas. Finally, and even more absurdly, we could not conceive of past sensations or anything that does not now exist. For all that is immediately before the mind is sensations that exist presently. Thus, we could not even conceive of qualities, bodies, minds, and things that do not now exist. But this is absurd, since it is obvious that we do think of all these things and even of things that have never existed. The solution, Reid suggested, is to abandon the theory of ideas and seek a better one.
Many have thought Reid was unfair to Hume and misinterpreted him. Reid’s Inquiry was presented to Hume by Dr. Blair in manuscript form, however, and in reply Hume does not at all suggest that he has been misinterpreted or handled unfairly. Whatever the merits of Reid’s criticism of Hume, it was the study of the consequences of Hume’s philosophy that accounts for Reid’s central doctrine of the human faculties and their first principles. Faculties are innate powers, among them the powers of conception and conviction. Reid’s strategy in reply to Hume is to build a nativist theory of conception on the failure of Hume’s theory of ideas. Where the theory of ideas, the doctrine of impressions and ideas, fails to account for our conception of something, of qualities, bodies, minds, past things, nonexistent things, Reid hypothesizes that our conceptions originate from a faculty of the mind, i.e., from an innate power of conception. This line of argument reflects Reid’s respect for Hume, whom he calls the greatest metaphysician of the age, because Hume drew forth the consequences of a theory of conception, which we might call associationism, according to which all our conceptions result from associating sensations. Where the associationism of Hume failed, Reid hypothesized that conceptions arise from innate powers of conception that manifest themselves in accordance with original first principles of the mind. The resulting hypotheses were not treated as a priori necessities but as empirical hypotheses. Reid notes, therefore, that there are marks by which we can discern the operation of an innate first principle, which include the early appearance of the operation, its universality in mankind, and its irresistibility. The operations of the mind that yield our conceptions of qualities, bodies, and minds all bear these marks, Reid contends, and that warrants the conclusion that they manifest first principles. It should be noted that Reid conjectured that nature would be frugal in the implantation of innate powers, supplying us with no more than necessary to produce the conceptions we manifest. Reid is, consequently, a parsimonious empiricist in the development of his nativist psychology. Reid developed his theory of perception in great detail and his development led, surprisingly, to his articulation of non-Euclidean geometry. Indeed, while Kant was erroneously postulating the a priori necessity of Euclidean space, Reid was developing non-Euclidean geometry to account for the empirical features of visual space. Reid’s theory of perception is an example of his empiricism. In the Inquiry, he says that sensations, which are operations of the mind, and impressions on the organs of sense, which are material, produce our conceptions of primary and secondary qualities. Sensations produce our original conceptions of secondary qualities as the causes of those sensations. They are signs that suggest the existence of the qualities. A sensation of smell suggests the existence of a quality in the object that causes the sensation, though the character of the cause is otherwise unknown. Thus, our original conception of secondary qualities is a relative conception of some unknown cause of a sensation. Our conception of primary qualities differs not, as Locke suggested, because of some resemblance between the sensation and the quality (for, as Berkeley noted, there is no resemblance between a sensation and quality), but because our original conceptions of primary qualities are clear and distinct. The sensation is a sign that suggests a definite conception of the primary quality, e.g. a definite conception of the movement of the object, rather than a mere conception of something, we know not what, that gives rise to the sensation.
These conceptions of qualities signified by sensations result from the operations of principles of our natural constitution. These signs, which suggest the conception of qualities, also suggest a conception of some object that has them. This conception of the object is also relative, in that it is simply a conception of a subject of the qualities. In the case of physical qualities, the conception of the object is a conception of a material object. Though sensations, which are activities of the mind, suggest the existence of qualities, they are not the only signs of sense perception. Some impressions on the organs of sense, the latter being material, also give rise to conceptions of qualities, especially to our conception of visual figure, the seen shape of the object. But Reid can discern no sensation of shape. There are, of course, sensations of color, but he is convinced from the experience of those who have cataracts and see color but not shape that the sensations of color are insufficient to suggest our conceptions of visual figure.
His detailed account of vision and especially of the seeing of visual figure leads him to one of his most brilliant moments. He asks what sort of data do we receive upon the eye and answers that the data must be received at the round surface of the eyeball and processed within. Thus, visual space is a projection in three dimensions of the information received on the round surface of the eye, and the geometry of this space is a non-Euclidean geometry of curved space. Reid goes on to derive the properties of the space quite correctly, e.g., in concluding that the angles of a triangle will sum to a figure greater than 180 degrees and thereby violate the parallels postulate. Thus Reid discovered that a non-Euclidean geometry was satisfiable and, indeed, insisted that it accurately described the space of vision (not, however, the space of touch, which he thought was Euclidean). From the standpoint of his theory of perceptual signs, the example of visual figure helps to clarify his doctrine of the signs of perception. We do not perceive signs and infer what they signify. This inference, Reid was convinced by Hume, would lack the support of reasoning, and Reid concluded that reasoning was, in this case, superfluous. The information received on the surface of the eye produces our conceptions of visual figure immediately. Indeed, these signs pass unnoticed as they give rise to the conception of