CHAPTER IX

THE PRINCIPLE OF MOVEMENT IN LIVING BEINGS WHAT IT IS NOT

             SINCE THE SOUL IS DEFINED BY TWO POWERS FOUND in animate beings, the one, discernment, the work of intellect and sensation, the other, movement by local motion; and as a certain amount has been decided about sensation and understanding, we must now consider what can be the moving factor in the soul: whether this is a single part of it, separate either spatially or by definition, or the whole soul; and, if it is a part of the soul, whether it is a special part other than those generally acknowledged and already dealt with, or some one among these.§ 795

             A difficulty at once arises as to how it is possible to speak at all of parts of the soul, or to say how many they are. For in one way their number seems to be infinite and not merely, as some say, the reasoning, the irascible and the concupiscible [parts]; or as others say, the rational and irrational.§§ 796-7

             For according to the differences by which these are distinguished, other parts seem to show greater diversity than [we see] in those just mentioned. In particular: the vegetative part, which is in plants and all living things; and the sensitive, which one cannot easily place among either the irrational or the rational elements.§§ 798-800

             Further, there is the imaginative power, which seems in essence to be quite different from any other. With which of these others it is identical, or from which it differs, are difficult problems: if one is to suppose that parts of the soul are separate.§ 801

             Furthermore, there is the appetitive faculty, which, both by its notion and as a capacity, seems to be diverse from all others; and it would be unreasonable to split this up. For will operates in the rational part, desire and anger in the irrational; and if the soul is in three parts, appetition will be in each.§§ 802-6

             But to come to the matter that is now before us: what is it that moves the animal by local motion? For the movement which is in all animals, by which they grow and decay, would certainly seem to be due to the principle of generation and nutrition. Of respiration and exhalation, sleep and waking, we must treat later on: these also raise many difficulties.§ 807

             But of local motion: what, we must consider, is it that moves an animal from place to place? It is obviously not the vegetative power. For a movement of this sort is always directed towards an end, and is accompanied by phantasm or desire. For nothing moves unless with desire or dislike, except under compulsion.§ 808

             Further: plants would also move about, and would possess some organic part suitable for such movement.§ 809

             Likewise, it is not the sensitive power. Many animals endowed with sensation are fixed and motionless all their lives. Yet if Nature does nothing in vain and is never deficient in what is necessary (save in imperfect or injured specimens; but such animals are complete and not defective, and there is proof of this in that they reproduce, grow and decay), it follows that they should also have the parts requisite for moving about.§§ 810-11

             Nor is the reasoning faculty or what is called intellect the moving power; for the speculative function does not consider in view of action, and has nothing to say about the avoidable or the desirable. But movement is always the avoidance or the pursuit of something. Nor, even when it does consider something of this kind, does it at once command pursuit or avoidance--for instance, it often thinks about some object of terror or delight, but without enjoining fear; though the heart moves, or, in the case of something pleasant, some other part.§§ 812-15

             Further; when the mind and the understanding do command avoidance or pursuit, the soul does not move [accordingly] but acts according to desire, as in the case of the incontinent. And in general we observe that one who knows the art of medicine does not [necessarily] heal,--as though it were the function of some power other than science to act according to science.§ 816

             But not even appetition imperates such movement as this. For the self-restrained do not act according to their desires even while they are actually wanting and desiring; instead they obey reason.§ 817