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:

Though known mainly for his epistemology, Reid is also noted for his views in the theory of action and the metaphysics of personal identity. Reid held an incompatibilist or libertarian notion of freedom, holding that we are capable of free actions of which we are the cause, and for which we are morally appraisable. Regarding personal identity, he rejected Locke's account that self-consciousness in the form of memory of one's experiences was the basis of a person's being identical with herself over time. Reid held that continuity of memory was neither necessary nor sufficient to make one numerically the same person at different times.

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

Until recently the standard edition of the Inquiry and the Essays has been the sixth edition of William Hamilton (ed.), Edinburgh: Maclachlan and Stewart, 1863. A new critical edition of these titles, plus correspondence and other important material, is being brought out by Edinburgh University Press as The Edinburgh Edition of Thomas Reid. An accessible selection from Hamilton's 6th ed. is Thomas Reid's Inquiry and Essays, ed. Ronald Beanblossom and Keith Lehrer, Indianapolis, IN: Hackett, 1983.