Now in order for a thing to be done for an end, some knowledge of the end is necessary.  Therefore, whatever so acts or is moved by an intrinsic principle, that it has some knowledge of the end, has within itself the principle of its act, so that it not only acts, but acts for an end.  On the other hand, if a thing has no knowledge of the end, even though it have an intrinsic principle of action or movement, nevertheless the principle of acting or being moved for an end is not in that thing, but in something else, by which the principle of its action towards an end is not in that thing, but in something else, by which the principle of its action towards an end is imprinted on it.  Wherefore such like things are not said to move themselves, but to be moved by others.  But those things which have a knowledge of the end are said to move themselves because there is in them a principle by which they not only act but also act for an end.  And consequently, since both are from an intrinsic principle, to wit, that they act and that they act for an end, the movements of such things are said to be voluntary:  for the word “voluntary” implies that their movements and acts are from their own inclination.  Hence it is that, according to the definitions of Aristotle, Gregory of Nyssa, and Damascene [See Objection 1], the voluntary is defined not only as having “a principle within” the agent, but also as implying “knowledge.”  Therefore, since man especially knows the end of his work, and moves himself, in his acts especially is the voluntary to be found.

Reply Obj. 1:  Not every principle is a first principle.  Therefore, although it is essential to the voluntary act that its principle be within the agent, nevertheless it is not contrary to the nature of the voluntary act that this intrinsic principle be caused or moved by an extrinsic principle:  because it is not essential to the voluntary act that its intrinsic principle be a first principle.  Yet again it must be observed that a principle of movement may happen to be first in a genus, but not first simply:  thus in the genus of things subject to alteration, the first principle of alteration is a heavenly body, which nevertheless is not the first mover simply, but is moved locally by a higher mover.  And so the intrinsic principle of the voluntary act, i.e. the cognitive and appetitive power, is the first principle in the genus of appetitive movement, although it is moved by an extrinsic principle according to other species of movement.

Reply Obj. 2:  New movements in animals are indeed preceded by a motion from without; and this in two respects.  First, in so far as by means of an extrinsic motion an animal’s senses are confronted with something sensible, which, on being apprehended, moves the appetite.  Thus a lion, on seeing a stag in movement and coming towards him, begins to be moved towards the stag.  Secondly, in so far as some extrinsic motion produces a physical change