All of Thomas Reid’s thought seems to grow out of his resistance to the skeptical conclusions of Berkeley and Hume. On the one hand, Reid is an effective critic, because he understands their arguments so well. By his own account, he started out as a convinced Berkeleyite and lost his enthusiasm only when he saw the skeptical scorched earth left by Hume. Instead of Berkeley’s real (albeit nonmaterial) and known world, with Hume there is not only no external world, there are no minds, no necessary relations of cause and effect, no rational inductive inferences, etc. Reid decided there must be something wrong, and identifies the problem as the theory of ideas (or “system of ideas” or “ideal system”), which he attributes to Descartes, Malebranche, and Locke (10; information on page references is given at the conclusion of this essay). He argues that Berkeley’s and Hume’s reasoning is mostly correct, derived by pursuing the theory of ideas more consistently than Descartes, Malebranche, and Locke had done. Thus, the extreme conclusions of Berkeley and Hume were due not to errors of reasoning but to errors of their starting points.

On the other hand, Reid’s effectiveness is undercut, in my view, by his failure to supply a convincing alternative to the theory of ideas. He should not be blamed for this. After all, it took philosophy more than 200 years after Reid to resolve the errors of the theory of ideas and put the philosophy of perception on a new footing. In my view, this has now been done with Intentionalism, and we shall see that Reid made promising steps in this direction. But he doesn’t go far enough, and as a result, he does not produce a fully convincing account of perception. He also fails to produce a plausible account of the contribution of the senses to conceptual thought, as the theory of ideas claims to do. Finally, I think Reid’s appeal to “common sense” is another reason he failed to convince. The notion of the authority of common sense—as a set of supposedly unchallengeable, quasi-axiomatic tenets, such as that there is an external world that we know by sense-perception—is fundamentally dogmatic and anti-intellectual, and it’s no surprise that philosophers generally denigrate it.

Thus, although Reid produced trenchant criticisms of the theory of ideas, these did not get the attention they should have. Every theory has problems and inadequacies. Accordingly, thinkers do not abandon a theory merely because it is in difficulties and arguably falsified or refuted. Rather, people abandon a theory when they have a better theory. Now that this is in our grasp, Reid suddenly looks a lot smarter than he used to.