In the Fourth Meditation, Descartes introduces a distinction between two basic faculties of the mind: the intellectus or intellect (translated as “entendement” in the French version of 1647 and sometimes as “understanding” in English translations), by which we perceive the ideas or propositions that are the subjects of possible judgments, and the will, by which we judge that propositions are true or false (AT VII 56, CSM II 39). We are exercising the intellect whenever we perceive ideas, whether they derive from sense experience or imagination or are instead innate ideas apprehended independently of the senses in what he calls either “pure understanding” (pura intellectio) (AT VII 15, 72; CSM II 11, 50) or perceiving by the “intellect alone” (solo intellectu) (AT VII 34, CSM II 22). For Descartes, the intellect is not a separate faculty from sensation, as in Scholasticism. In his new conception of the mind as thought or consciousness, sensation comprises both a bodily process and a mental having of ideas, and thus it too involves an exercise of the intellect, though not of course a pure exercise of intellect alone.

The distinction between intellect and will builds on the Third Meditation's distinction between two kinds of thoughts: ideas, which are “as it were the images of things,” and thoughts containing “certain additional forms,” namely acts of will, including judgments, that involve various attitudes we can take up toward ideas, such as fearing, wishing, affirming, or denying (AT VII 37, CSM II 25–26). As this second distinction makes plain, intellect and will, though separate faculties, are also geared to work together in that “when we direct our will towards something, we always have some understanding of some aspect of it” (AT VII 377, CSM II 259). Ideally, we ought in theoretical inquiry to affirm only those ideas which we clearly and distinctly perceive to be true, ideas that, when we attend to why they are indubitable, compel our assent (see reason). Error arises only when we let our will acquiesce to the less demanding products of the understanding.