Descartes’ doctrine of the indivisibility of the mind constitutes one of his two main arguments for the mind-body dualism in the Meditations (AT VII 85–86, CSM II 59). Breaking with Plato's and Aristotle's division of the soul into rational and irrational parts, Descartes insists that the human mind, in being a substance, is one and undivided, “single and complete” (ibid.). “There is within us but one soul and this soul has within it no diversity of parts” (AT XI 364, CSM I 346). Yet Descartes holds that the unity and simplicity of the human mind allows for a variety of “faculties” or powers. Far from being parts of the mind or entities distinct from it, these faculties are different powers, dispositions, or capacities of one and the same mind. As Descartes writes, “the term ‘faculty’ denotes nothing but a potentiality” (AT VIIIB 361, CSM I 305). Descartes believes that it is an error to regard the mind's faculties as its separated parts or as subminds, since neither of them performs the operations of thinking: “It is one and the same mind that wills, and understands and has sensory perception.” Intellect, will, sense perception, imagination, and memory are all different faculties or potentialities of a single, unified thinking substance (AT VII 86, CSM II 59; cf. AT III 371–72, CSMK 182). Of these faculties, the intellect and the will are purely mental, whereas sense perception, imagination, and memory depend also on the body. Descartes accordingly insists that the conflicts usually supposed to obtain between rational and irrational parts or forces of the soul are conflicts between rational volitions originating in the soul and bodily movements, each pushing the pineal gland in contrary directions (AT XI 364, CSM I 345). Our failure to properly distinguish the functions of the soul from those of the body leads us to misconceive the different functions of the soul as different subpersons playing opposite roles. However, “it is to the body alone that we should attribute everything that can be observed in us to oppose our reason” (AT XI 365, CSM I 346).

In Descartes’ mature philosophy of mind, the two main faculties or powers of the mind are the intellect, which Descartes characterizes as the “faculty of knowledge” (facultas cognoscendi), and the will, which is the “faculty of choosing” (facultas eligendi) (AT VII 56, CSM II 39).