De Anima

The soul in animals has been defined in virtue of two faculties, not only by its capacity to judge, which is the function of thought and perception, but also by the local movement which it imparts to the animal. Assuming the nature of sensation and intellect to have been so far determined, we have now to consider what it is in the soul which initiates motion: whether it is some one part of the soul, which is either locally separable or logically distinct, or whether it is the whole soul: and again, if a separate part, whether it is a special part distinct from those usually recognised and from those enumerated above, or whether it coincides with some one of these.

A question at once arises in what sense it is proper to speak of parts of the soul and how many there are. For in one sense there appear to be an infinite number of parts and not merely those which some distinguish, the reasoning, passionate and concupiscent parts, for which others substitute the rational and the irrational. For, if we examine the differences on which they base their divisions, we shall find that there are other parts separated by a greater distance than these; namely, the parts which we have just discussed, the nutritive, which belongs to plants as well as to all animals, and the sensitive, which cannot easily be classed either as rational or irrational. Imagination, again, is logically distinct from them all, while it is very difficult to say with which of the parts it is in fact identical or not identical, if we are to assume separate parts in the soul. Then besides these there is appetency, which would seem to be distinct both in concept and in capacity from all the foregoing. And surely it is absurd to split this up. For wish in the rational part corresponds to concupiscence and passion in the irrational. And, if we make a triple division of soul, there will be appetency in all three parts.

To come now to the question at present before us, what is it that imparts to the animal local movement? For as for the motion of growth and decay, which is found in all animals, it would seem that this must be originated by that part of soul which is found in all of them, the generative and nutritive part. Inspiration and expiration of breath, sleep and waking, subjects full of difficulty, call for subsequent enquiry. But to return to locomotion, we must enquire what it is that imparts to the animal progressive motion. That it is not the nutritive faculty is clear. For this motion is always directed to an end and is attended either by imagination or by appetency. No animal, which is not either seeking or avoiding something, moves except under compulsion. Moreover, if it were the nutritive faculty, plants also would be capable of locomotion and thus would have some part instrumental in producing this form of motion. Similarly it is not the sensitive faculty, since there are many animals which have sensation and yet are throughout their lives stationary and motionless. If, then, nature does nothing in vain and, except in mutilated and imperfect specimens, omits nothing that is indispensable, while the animals we are considering are fully developed and not mutilated—as is shown by the fact that they propagate their kind and have a period of maturity and a period of decline,—it follows that, if locomotion was implied in sensation, they would have had the parts instrumental to progression. Nor, again, is it the reasoning faculty or what is called intellect that is the cause of motion. For the speculative intellect thinks nothing that is practical and makes no assertion about what is to be avoided or pursued, whereas motion always implies that we are avoiding or pursuing something. But, even if the mind has something of the kind before it, it does not forthwith prompt avoidance or pursuit. For example, it often thinks of something alarming or pleasant without prompting to fear; the only effect is a beating of the heart or, when the thought is pleasant, some other bodily movement. Besides, even if the intellect issues the order and the understanding bids us avoid or pursue something, still we are not thereby moved to act: on the contrary, action is determined by desire; in the case, for instance, of the incontinent man. And generally we see that, although a man possesses a knowledge of medicine, it does not follow that he practises; and this implies that there is something else apart from the knowledge which determines action in accordance with the knowledge. Nor, again, is it solely appetency on which this motion depends. The continent, though they feel desire, that is appetite, do not act as their desires prompt, but on the contrary obey reason.