Descartes used the term “perception” (Latin perceptio, French perception) in several related ways. In the most general sense, expressed in a formal definition in the Principles of Philosophy, “perception” is any operation of the intellect (as contrasted with the will), comprising sensation or sense perception (sentire), imagination, and pure intellect or pure understanding (AT VIIIA 17, CSM I 204). In this general sense, to perceive is to be aware of or to grasp ideas or mental contents and perhaps, by virtue of such contents, to cognize mind-independent objects and their properties. Descartes applies this notion of perception to a variety of mental contents, including obscure and confused sensations, clear and distinct ideas (see clarity and distinctness), images in imagination, and purely intellectual ideas of extension, thought, and simple natures, as well as to principles such as “what is done cannot be undone” (AT VII 145, CSM II 104) and to everyday propositions, such as that wax is present or men are in the square. The latter act of perception is complex, involving the sensory experiences of human-shaped figures and the judgment that they are men (AT VII 32, CSM II 21).

For discovering and establishing the principles of Descartes’ metaphysics, the clear and distinct perceptions of the pure intellect are foundational. His Meditations can be read as preparing a reader hitherto immersed in sensory experience to enter a special kind of cognitive state: the perception of metaphysical truths concerning the essences of things (AT VII 9, 171–72, 178, 440–47; CSM II 8, 121, 126, 296–301). Building on the claim in the Second Meditation that he perceives the existence of a thing that thinks from the experience of doubt or other thoughts (the cogito reasoning), Descartes guides the reader to seek a purely intellectual apprehension of God in the Third and Fifth Meditations, an intellectual perception of extension and of geometrical essences in the Fifth and Sixth Meditations, and the clear and distinct perception of mind and body as distinct kinds of substance in the Sixth Meditation.

For these purposes, Descartes has the reader turn away from sense perception and imagination. He excludes sense perception as a first basis for metaphysics, ceding authority to the intellect in perceiving “the essential nature of bodies” (AT VII 83, CSM II 58).