[9] Shortly after the publication of his first book, he was given the prestigious Professorship of Moral Philosophy at the University of Glasgow when he was called to replace Adam Smith.

By contrast, Reid claimed that the foundations upon which our sensus communis are built justify our belief that there is an external world.

Hume responded that the work "is wrote in a lively entertaining manner," although he found "there seems to be some Defect in Method", and he criticized Reid's doctrine for implying the presence of innate ideas.

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.

Reid's answer to Hume's sceptical 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.

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.

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.

Reid also made positive arguments based in phenomenological insight to put forth a novel mixture of direct realism and ordinary language philosophy.

If, says Reid, the child needed to understand immediately the conceptual content of the words it hears, it would never learn to speak at all.

While we become aware of an object through the senses, the content of that perception is not identical with the sum total of the sensations caused in our consciousness.

Thus, while we tend to focus on the object perceived, we pay no attention to the process leading from sensation to perception, which contains the knowledge of the thing as real.

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.

[15] 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".

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 because of the attention given to Reid by contemporary philosophers, in particular philosophers of religion in the school of Reformed epistemology such as William Alston,[17] Alvin Plantinga, and Nicholas Wolterstorff,[18] 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).

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.

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...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.

[20] Reid also argued that the operation of our mind connecting sensations with belief in an external world is accounted for only by an intentional Creator.

In his natural religion lectures, Reid provides five arguments for the existence of God, focusing on two mainly, the cosmological and design.

Reid loved and frequently used Samuel Clarke's cosmological argument, which says, in short that the universe either has always been, or began to exist, so there must be a cause (or first principle) for both (Cuneo and Woudenberg 242).

This can't be known by experience, fitting with the casual excellence principle, but the cause can be seen in works of nature (Cuneo and Woudenberg 241).