But this reasoning may be met by the following reply on behalf of Plato's view. -- There is no difficulty, it will be said, in mover and moved having the same act, notwithstanding their difference in being: for motion is at once the act of the moving force, from which it is, and the act of the thing moved, in which it is. Thus then, on Plato's theory, the aforesaid activities may be common to soul and body, belonging to the soul as the moving force, and to the body as the thing moved. But this explanation cannot hold for the following reasons.
1. As the Philosopher proves (De Anima, II), sensation results by the sentient subject being moved or impressed by external sensible things: hence a man cannot have a sensation without some external sensible thing,* as nothing can be moved without a mover. The sensory organ therefore is moved and impressed in sensation, but that is by the external sensible object. What receives the impression is the sense, as is evident from this, that senseless things do not receive any such manner of impression from sensible objects. The sense therefore is the passive power of the sensory organ. The sentient soul therefore in sensation does not play the part of mover and agent, but is that principle in the subject impressed, in virtue of which the said subject lies open to the impression. But such a principle cannot be different in being from the subject impressed. Therefore the sentient soul is not different in being from the animated body.
2. Though motion is the common act of moving force and object moved, still it is one activity to impart motion and another to receive motion: hence the two several categories of action and passion. If then in sensation the sentient soul stands for the agent, and the body for the patient, there will be one activity of the soul and another of the body. The sentient soul therefore will have an activity and proper motion of its own: it will have therefore its own subsistence: therefore, when the body perishes, it will not cease to be.* Thus sentient souls, even of irrational animals, will be immortal; which seems improbable, although it is not out of keeping with Plato's opinion.* But this will be matter of enquiry further on (Chap. LXXXII).
3. A body moved does not take its species according to the power that moves it. If therefore the soul is only united to the body as mover to moved, the body and its parts do not take their species from the soul: therefore, when the soul departs, the body and the parts thereof will remain of the same species. But this is manifestly false: for flesh and bone and hands and such parts, after the departure of the soul, do not retain their own names except by a façon de parler;* since none of these parts retains its proper activity, and activity follows species. Therefore the union of soul and body is not that of mover with moved, or of a man with his dress.
6. If the soul is united with the body only as mover with moved, it will be in the power of the soul to go out of the body when it wishes, and, when it wishes, to reunite itself with the body.* That the soul is united with the body as the proper form of the same, is thus proved. That whereby a thing emerges from potential to actual being, is its form and actuality. But by the soul the body emerges from potentiality to actuality: for the being of a living thing is its life: moreover the seed before animation is only potentially alive, and by the soul it is made actually alive:* the soul therefore is the form of the animated body. Again: as part is to part, so is the whole sentient soul to the whole body. But sight is the form and actuality of the eye:* therefore the soul is the form and actuality of the body.
2.56, 69 : How a Subsistent Intelligence may be United with a Body, with a Solution of the Arguments alleged to prove that a Subsistent Intelligence cannot be United with a Body as its Form
2.58 : That Vegetative, Sentient, and Intelligent are not in Man Three Souls