The “One Soul-Many Powers” Argument

The dominant view among late medieval thinkers was that according to which human beings have one soul and that this soul processes information about cognitive objects (perceptual, intellectual) by means of different powers (or faculties or capacities). The nature of the relation between these powers and the soul is complex and subject to much disagreement: the main issue arises not from the existence of many powers organised into sets, i.e., as powers of certain parts of the soul, but how they come together to form a unified soul and what the unity of the soul means. One way to understand this is to take the unity of the soul as an answer to the question about the nature of the relation between the powers that make possible the unity of cognitive experience, that is, the cognition of one object under its many aspects. I call this interpretation the 'one soul, many powers' argument. I focus on one version of this argument, which Dominik Perler identifies in Thomas Aquinas. For Aquinas, a strong ontological unity of the soul – the existence in the soul of only one substantial form – is necessary to justify the functional unity of the powers that underlie the unity of experience. According to this view, an account of the human soul as a plurality of forms cannot properly explain how different powers come together in the constitution of a unified cognitive experience. I show that this is not so and that at least certain versions of pluralism (in this case, that of Peter John Olivi) are compatible with the functional unity of powers that explains the unified account of experience.