Epistemology

The Cartesian meditative project invites a skeptical worry about the existence of the external world, for when the mind withdraws from the senses, it cuts itself off from the material world altogether. Indeed, in order for the narrator of the Meditationes to achieve his goal of razing the edifice of all of his former opinions, he must deliberately cultivate a radical form of skepticism, accepting the possibility that his ideas do not represent any independent material reality but may instead be the work of a "malicious demon." By the end of the Meditationes, the existence of the external world is supposed to have been established through a proof of the existence of God, which is in turn supposed to yield results about the reliability of the meditator's cognitive faculties (on the testimony of which the meditator supposes that there is a world independent of his mind).

Philosophers after Descartes continued to struggle with the fact that the Cartesian framework compels one to conclude that perception does not put us in direct contact with the outside world. One has contact with the world only through a "veil of ideas," and it seems one can only infer the presence of an external, independent, material world.

George Berkeley (1685–1753) tried to defend common sense against this skeptical worry by advancing an idealist account of perception in Three Dialogues between Hylas and Philonous (1713). His account is "idealist" because it argues that there is no mind-independent reality. At bottom Berkeley disputes the coherence of the Cartesian conception of material substance. Building on the premise that one cannot represent any physical object without having "ideas" of its qualities, he argues that objects are identified by the constant conjunction of certain ideas: "Take away the sensations of softness, moisture, redness, tartness, and you take away the cherry. Since it is not a being distinct from sensations; a cherry, I say, is nothing but a congeries of sensible impressions" (p. 130). The cause of these ideas cannot be material substance, Berkeley argues, but only the mind of God. Berkeley's curious defense of common sense comes down to a denial that there is any reality independent of "mind" at all.

The Scottish philosopher Thomas Reid (1710–1796) saw more clearly than any of his predecessors that skepticism about the "veil of ideas" is the unavoidable result of any adherence to the dominant modern theory of perception. He argues so at length in An Inquiry into the Human Mind, on the Principles of Common Sense (1764) and Essays on the Intellectual Powers of Man (1785) and offers a new account of perception that is not based on the modern "way of ideas." Although Reid was largely neglected by historians of philosophy for most of the twentieth century, his account of perception has received greater attention since the mid-1990s as a source of philosophical insight in its own right.