Two ways of speaking of acting-upon and being-acted upon 324a16-24:
1. The substratum: sometimes we say that it is the substratum which is acted upon, e.g., a man is restored to health, or that it is the man who causes heat.
2. The contraries: sometimes we say that it is the contrary which is acted upon, e.g., the cold is heated or the hot causes heat.
Two Senses of Mover
1. The First Mover (τὸ πρῶτον κινοῦν, 324a30) is unmoved, e.g., the doctor, since those things which have not the same matter act without being themselves affected (ποιεῖ ἀπαθῆ ὄντα, 324a34). The art of healing, while it causes health, is not itself acted upon by that which is being healed. The art of healing does not have the same matter as the patient's body.
2. The Last Mover (τὸ ἔσχατον) always causes motion by itself being moved (ἀεὶ κινεῖν κινούμενον, 324a32). The food (σιτίον), while it acts, is acted upon in some way, since it is heated or cooled at the same time (ἅμα, 324b3) in some way.
