Epistemology

The early modern theory of sense perception combined aspects of the mechanistic conception of nature with the Cartesian dualism of mind and body. Sense representations are "ideas," things proper to the immaterial mind; they are caused by qualities that are attributed to material bodies. It was thought to be possible, at least in principle, to give a mechanistic account of how a particular arrangement of the corpuscular micro-structure brings about—through the motion of bodies—a certain "impression" on our sensory organs. But without overcoming the problematic of Cartesian dualism, no satisfying explanation of how some such physical impact (an "impression") could yield something mental (an "idea"). John Locke (1632–1704), an empiricist philosopher who ambivalently accepted aspects of Cartesian dualism, claimed in his Essay concerning Human Understanding (1690) that one must "take notice" of impressions in order to enjoy ideas of sense or have any sense perception at all (book 2, chapter 5). This capacity of the mind to "take notice" of impressions thus figures as an unexplained explainer in Locke's philosophy.

Descartes and Locke both thought that unnoticed judgments play an important role in sense perception. This again has to do with the Cartesian conception of the mind: the mind is transparent to itself, having infallible awareness of its own contents, or "ideas." People err only when they make judgments about the world on the basis of these ideas. To account for this, Descartes distinguishes between the understanding, which is simply an active capacity to be conscious of the mind's ideas, and the will, which affirms or denies that certain relations of ideas represent states of affairs in the world. For Descartes, judgment involves an act of the will. Although Locke denies Descartes's view about the role that the will plays in judgment, he agrees that unnoticed and habitual judgments play an important role in perception. When looking at a sphere of a uniform color, Locke claims, "the Idea thereby imprinted on our Mind, is of a flat Circle variously shadow'd"; but a habitual judgment "alters the Appearances into their Causes" so that what is perceived is "a convex Figure, and an uniform Colour" (p. 145).