Didascalicon, Book I (chapters 2-13): the origin of the arts and the division of philosophy
Chapter IV. That the power of the soul is threefold, and that man alone is endowed with reason
A threefold power of the soul is found altogether in the animating of bodies. One of these supplies only the life of the body, so that it subsists by being born and nourished. Another rests on the judgment of sensing; the third is founded on the power of the mind and on reason. The office of the first of these is to be at hand for the creating, feeding and nourishing of bodies, but it affords no judgment of reason or of sense. This is the soul of herbs and trees, and of whatever is held fixed by roots in the earth. The second is composed and joined, taking the first to itself and making it a part, and it receives the judgment of what it can grasp, in many forms. For every animal that is alive with sense is also born, and fed, and nourished. The senses are five, and increase up to the number of five; thus whatever is merely nourished does not also sense. But whatever can sense is also nourished, and the first power of the soul, namely of being born and nourished, is shown to be subject to it.
Where sense is present, living things not only take in the forms of things by which they are struck in a sensible body, but when the sense is withdrawn they also retain images of the forms perceived, and make memory; and according as each animal is able, it keeps them longer or more briefly. But they receive these imaginations confused and indistinct, so that they can effect nothing from their combination and composition; and therefore they can indeed remember, but are not able to gather up and recall a forgetfulness that has set in. Of the future they have no knowledge. The third power of the soul draws along with it the former two, of nourishing and of sensing, and uses them as servants and obedient subjects; it is wholly established in reason, and is occupied either in the firmest conclusion about present things, or in the understanding of absent things, or in the inquiry into unknown things. This is at hand to the human race alone, which not only takes in senses and imaginations in a perfect and not disordered way, but also, by the full act of understanding, explains and confirms what the imagination has suggested. And so (as has been said) for this divine nature it is not enough to know only what it comprehends as subject to the senses, but it can also give names to things conceived by imagination from sensible things and to absent things, and what it comprehends by understanding in reason it also discloses by the imposition of words. This too is proper to that nature, that by the things which are known to it it investigates the unknown. And it must know not only that each thing is, but also what it is, and of what kind it is, and also why it is.
This threefold power of the soul has fallen, as has been said, to the nature of men alone. The power of this soul is not without movements of understanding, since in these four it properly exercises the power of reason. For either it inquires whether something is; or, as it has been established that it is, it asks what it is; and if it also possesses by reason the knowledge of both, it investigates by reason of what kind each thing is, and in it searches out the other moments of accidents. When these are known, it asks why it is so, and likewise investigates by reason. Since, then, this is the act of the human mind, to be always occupied in the comprehension of present things, or the understanding of absent things, or the inquiry into and discovery of unknown things, there are two ends to which the whole labor of the reasoning soul is devoted: one, that it may know the natures of things by the method of inquiry; the other, that it may come first to that knowledge which the gravity of mortal life may afterwards put into practice.