Most medieval thinkers assume that the human soul has several faculties or powers: basic faculties such as digestion or growth, more elaborate faculties such as movement, vision, or imagination, and the characteristically human faculties of will and intellect. This was the mainstream position, but it was not left unquestioned in the later Middle Ages and in early modern philosophy. Several nominalists, for instance, argue that the powers of the soul are nothing but different names for the soul itself, as it is active in different ways. Later, in the seventeenth century, mechanistic philosophers such as René Descartes claim that there is no real distinction between power and act, nor between soul and powers. Descartes reserves the term ‘soul’ for the mind, and so reduces the number of powers drastically; he claims that all lower powers, such as sense perception or imagination, are equivalent either to the mind or certain powers of the body. Even Thomistic authors of the sixteenth and seventeenth centuries, who usually defend the theory of the faculties, at times question the traditional set of faculties and reduce their number. Francisco Suárez, for example, holds that common sense, imagination, estimation, and memory are in fact one power, because all these functions can be attributed to one faculty.

Nevertheless, in spite of the criticisms voiced by nominalist and early modern philosophers, medieval faculty psychology itself was well supported by arguments that have their origin in Greek philosophy. In the Republic, for example, Plato proposes a threefold division of the soul into reason, spirit, and desire. He bases this theory on the fact that there are conflicts in the soul: we may desire an object and at the same time reject it, as when we desire to drink something but reject it because we think it is bad for us. This can be explained, he believes, only by assuming that the soul has distinct parts that can come into conflict with each other (435e–439d).