Descartes’ theory of sensible qualities is situated in his mathematical physics, which rejects Aristotle's naïve realism, according to which bodies actually have the colors, flavors, odors, hot, and cold we sense in them. Echoing Galileo's claim in The Assayer, that physics does not need to assume the reality of these qualities, Descartes argues that the only real properties of body are extension, shape, and motion. To make his case he denies the resemblance thesis, that the ideas of sensible qualities resemble the properties in bodies that cause them. Following Descartes, John Locke similarly rejects this thesis by means of the famous distinction between primary and secondary qualities of bodies.

While this much is generally agreed on, several interpretive issues concerning sensible qualities have arisen, including differing accounts of Descartes’ reasons for rejecting the resemblance thesis and his basis for claiming that sensory experience is obscure and confused. Other questions concern whether Descartes eliminates sensible qualities altogether from physics, the referents of sensible-quality terms, the metaphysical status of sensible qualities, and his analysis of real physical properties. (Note that Descartes’ theory of sensible qualities is not to be confused with his rejection of the Scholastic doctrine of real qualities. See Menn 1995 and quality, real.)

Throughout his writings, Descartes distinguishes the “sensible qualities” from the properties in bodies intelligible to the understanding. In the Principles of Philosophy IV.191–95, he reserves the term “quality” for the content of sensations produced in the mind by the five external senses: touch, taste, smell, hearing, and sight. Tactile qualities include hardness, heaviness, heat, and humidity; taste results in sensations of flavors; smell yields sensations of odors; from hearing arise sounds; and vision produces sensations of light and colors (AT VIIIA 318–19, CSM I 281–83). In the wax analysis in Second Meditation, he separates the sensible qualities from the essential properties of bodies. After stripping off the changeable features of the wax, he concludes that a clear and distinct grasp of body by the intellect reveals it to be “merely something extended, flexible and changeable.” Thus the coolness, fragrance, and color of the wax are merely ways it appears to perceivers under certain conditions (AT VII 30–32, CSM II 20–21).