The Reverend Thomas Reid, D.D. (7 May (26 April O.S.) 1710 – 7 October 1796), was a religiously trained Scottish philosopher, and a contemporary of David Hume, was the founder of the Scottish School of Common Sense, and played an integral role in the Scottish Enlightenment. The early part of his life was spent in Aberdeen, Scotland, where he created the 'Wise Club' (a literary - philosophical association) and graduated from the University of Aberdeen. He was given a professorship at King's College, Aberdeen, in 1752, where he wrote An Inquiry Into the Human Mind on the Principles of Common Sense (published in 1764). Shortly afterward he was given the prestigious Professorship of Moral Philosophy at the University of Glasgow when he was called to replace Adam Smith. He resigned from this position in 1781.

Reid believed that common sense (in a special philosophical sense of sensus communis) is, or at least should be, at the foundation of all philosophical inquiry. He disagreed with Hume, who asserted that we can never know what an external world consists of as our knowledge is limited to the ideas in the mind, and George Berkeley, who asserted that the external world is merely ideas in the mind. By contrast, Reid claimed that the foundations upon which our sensus communis are built justify our belief that there is an external world.

In his day and for some years into the 19th century, he was regarded as more important than David Hume. He advocated direct realism, or common sense realism, and argued strongly against the Theory of Ideas advocated by John Locke, René Descartes, and (in varying forms) nearly all Early Modern philosophers who came after them. He had a great admiration for Hume and had a mutual friend send Hume an early manuscript of his (Reid's) Inquiry. Hume responded that the "deeply philosophical" work "is wrote in a lively and entertaining matter," but that "there seems to be some defect in method," and criticized Reid for implying the presence of innate ideas.

His theory of knowledge had a strong influence on his theory of morals. He thought epistemology was an introductory part to practical ethics: When we are confirmed in our common beliefs by philosophy, all we have to do is to act according to them, because we know what is right. His moral philosophy is reminiscent of Latin stoicism in its emphasis on the agency of the subject and self - control. He often quotes Cicero, from whom he adopted the term "sensus communis". Reid's answer to Hume's skeptical and naturalist arguments was to enumerate a set of principles of common sense (sensus communis) which constitute the foundations of rational thought. Anyone who undertakes a philosophical argument, for example, must implicitly presuppose certain beliefs like, "I am talking to a real person," and "There is an external world whose laws do not change," among many other positive, substantive claims. For Reid, the belief in the truth of these principles is not rational; rather, reason itself demands these principles as prerequisites, as does the innate "constitution" of the human mind. It is for this reason (and possibly a mocking attitude toward Hume and Berkeley) that Reid sees belief in the principles of common sense as a litmus test for sanity. For example, in The Intellectual Powers of Man he states, “For, before men can reason together, they must agree in first principles; and it is impossible to reason with a man who has no principles in common with you.” One of the first principles he goes on to list is that “qualities must necessarily be in something that is figured, coloured, hard or soft, that moves or resists. It is not to these qualities, but to that which is the subject of them, that we give the name body. If any man should think fit to deny that these things are qualities, or that they require any subject, I leave him to enjoy his opinion as a man who denies first principles, and is not fit to be reasoned with.”

Reid also made positive arguments based in phenomenological insight to put forth a novel mixture of direct realism and ordinary language philosophy. In a typical passage in the Intellectual Powers he asserts that when he has a conception of a centaur, the thing he conceives is an animal, and no idea is an animal; therefore, the thing he conceives is not an idea, but a centaur. This point relies both on an account of the subjective experience of conceiving an object and also on an account of what we mean when we use words. Because Reid saw his philosophy as publicly accessible knowledge, available both through introspection and the proper understanding of how language is used, he saw it as the philosophy of common sense.

It has been claimed that his reputation waned after attacks on the Scottish School of Common Sense by Immanuel Kant (although Kant, only 14 years Reid's junior, also bestowed much praise on Scottish philosophy) and by John Stuart Mill. But Reid's was the philosophy taught in the colleges of North America during the 19th century and was championed by Victor Cousin, a French philosopher. Justus Buchler has shown that Reid was an important influence on the American philosopher Charles Sanders Peirce, who shared Reid's concern to revalue common sense and whose work links Reid to pragmatism. To Peirce, conceptions of truth and the real involve the notion of a community without definite limits (and thus potentially self - correcting as far as needed), and capable of a definite increase of knowledge. Common sense is socially evolved, open to verification much like scientific method, and constantly evolving, as evidence, perception, and practice warrant, albeit with a slowness that Peirce came only in later years to see, at which point he owned his "adhesion, under inevitable modification, to the opinion of... Thomas Reid, in the matter of Common Sense" (Peirce called his version "critical common - sensism"). By contrast, on Reid's concept, the sensus communis is not a social evolutionary product but rather a precondition of the possibility that humans could reason with each other.

Reid's reputation has revived in the wake of the advocacy of common sense as a philosophical method or criterion by G.E. Moore early in the 20th century, and more recently due to the attention given to Reid by contemporary philosophers, in particular philosophers of religion in the school of Reformed epistemology such as William Alston, Alvin Plantinga, and Nicholas Wolterstorff, seeking to rebut charges that theistic belief is irrational where it has no doxastic foundations (that is, where that belief is not inferred from other adequately grounded beliefs).

He wrote a number of important philosophical works, including Inquiry into the Human Mind on the Principles of Common Sense (1764, Glasgow & London), Essays on the Intellectual Powers of Man (1785) and Essays on the Active Powers of Man (1788). In 1844, Schopenhauer praised Reid for explaining that the perception of external objects does not result from the raw data that is received through the five senses:

Thomas Reid's excellent book, Inquiry into the Human Mind... affords us a very thorough conviction of the inadequacy of the senses for producing the objective perception of things, and also of the non-empirical origin of the intuition of space and time. Reid refutes Locke's teaching that perception is a product of the senses. This he does by a thorough and acute demonstration that the collective sensations of the senses do not bear the least resemblance to the world known through perception, and in particular by showing that Locke's five primary qualities (extension, figure, solidity, movement, number) cannot possibly be supplied to us by any sensation of the senses...

— The World as Will and Representation, Vol. II, Ch. 2