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 the 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.[citation needed] 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 asked him to correct the first manuscript of his (Reid's) Inquiry.

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