Encyclopedia of Philosophy: AQUINAS' PHILOSOPHY OF MIND

In treating of the human mind Aquinas had an exacting task: he wanted to show that it was possible to accept Aristotle's psychology without following Averroes in denying the immortality of the individual human soul. Like Bonaventure, Aquinas refused to accept the Arab philosophers' theory that human beings shared a common universal intellect. The intellect which sets human beings apart from other animals can be thought of, without too much distortion of Aquinas' thought, as the capacity for thinking those thoughts that only a language-user can think. This power was, for Aquinas, a faculty of the individual human soul. Following Aristotelian tradition, he distinguished between an active intellect and a receptive intellect; both of these, he insisted, were powers which each one of us possesses. The active intellect is the capacity to form universal ideas and to attain necessary truth. The receptive intellect is the storehouse of ideas and knowledge once acquired (see Plate 10).  According to Aquinas, the intellect acquires its concepts by reflection upon sensory experience; we have no innate ideas, nor, for everyday knowledge, do we receive any special divine illumination. Experience is necessary for human acquisition of concepts, but it is not sufficient; that is why we have a special conceptforming capacity, the active intellect. We need it, Aquinas thought, because the material objects of the world we live in are not, in themselves, fit objects for intellectual understanding. A Platonic Idea, universal, intangible, unchanging, unique, might be a suitable object for the intellect; but in our world there are no such things as Platonic Ideas, and if there are any in the mind of God that is no business of ours in our present lives. So, Aquinas concludes, we need a special power in order to create what he calls ‘actually thinkable objects' by abstracting ideas from our experience of the world. This power is the active intellect.