De Anima

The motive causes are apparently, at any rate, these two, either appetency or intelligence, if we regard imagination as one species of thinking. For men often act contrary to knowledge in obedience to their imaginings, while in the other animals there is no process of thinking or reasoning, but solely imagination. Both these, then, are causes of locomotion, intelligence and appetency. By intelligence we mean that which calculates the means to an end, that is, the practical intellect, which differs from the speculative intellect by the end at which it aims. Appetency, too, is directed to some end in every case: for that which is the end of desire is the starting point of the practical intellect, and the last stage in this process of thought is the starting point of action. Hence there is good reason for the view that these two are the causes of motion, appetency and practical thought. For it is the object of appetency which causes motion; and the reason why thought causes motion is that the object of appetency is the starting point of thought. Again, when imagination moves to action, it does not move to action apart from appetency. Thus there is one single moving cause, the appetitive faculty. For, had there been two, intelligence and appetency, which moved to action, still they would have done so in virtue of some character common to both. But, as a matter of fact, intellect is not found to cause motion apart from appetency. For rational wish is appetency; and, when anyone is moved in accordance with reason, he is also moved according to rational wish. But appetency may move a man in opposition to reason, for concupiscence is a species of appetency. While, however, intellect is always right, appetency and imagination may be right or wrong. Hence it is invariably the object of appetency which causes motion, but this object may be either the good or the apparent good. Not all good, however, but practical good: where by practical good we mean something which may not be good under all circumstances.

It is evident, then, that motion is due to the faculty of the soul corresponding to this object,—I mean what is known as appetency. But those who divide the soul into parts, if they divide it according to its powers and separate these from one another, will find that such parts tend to become very numerous: nutritive, sensitive, intelligent, deliberative, with the further addition of an appetent part: for these differ more widely from one another than the concupiscent does from the passionate. Now desires arise which are contrary to one another, and this occurs whenever reason and the appetites are opposed, that is, in those animals which have a perception of time. For intelligence bids us resist because of the future, while appetite has regard only to the immediate present; for the pleasure of the moment appears absolutely pleasurable and absolutely good because we do not see the future. Therefore, while generically the moving cause will be one, namely, the faculty of appetency, as such, and ultimately the object of appetency (which, without being in motion itself, causes motion by the mere fact of being thought of or imagined), numerically there is a plurality of moving causes.

Now motion implies three things, first, that which causes motion, secondly, that whereby it causes motion, and again, thirdly, that which is moved; and of these that which causes motion is twofold, firstly, that which is itself unmoved and, secondly, that which both causes motion and is itself moved. The unmoved movent is the practical good, that which is moved and causes motion is the appetitive faculty (for the animal which is moved is moved in so far as it desires, and desire is a species of motion or activity) and, finally, the thing moved is the animal. But the instrument with which desire moves it, once reached, is a part of the body: hence it must be dealt with under the functions common to body and soul. For the present, it may be enough to say summarily that we find that which causes motion by means of organs at the point where beginning and end coincide; as, for instance, they do in the hinge-joint, for there the convex and the concave are respectively the end and the beginning, with the result that the latter is at rest, while the former moves, convex and concave being logically distinct, but locally inseparable. For all animals move by pushing and pulling, and accordingly there must be in them a fixed point, like the centre in a circle, and from this the motion must begin. Thus, then, in general terms, as already stated, the animal is capable of moving itself just in so far as it is appetitive: and it cannot be appetitive without imagination. Now imagination may be rational or it may be imagination of sense. Of the latter the other animals also have a share.