Next in order we must discuss action and passion. The traditional theories on the subject are conflicting. For (i) most thinkers are unanimous in maintaining (a) that like is always unaffected by like, because (as they argue) neither of two likes is more apt than the other either to act or to suffer action, since all the properties which belong to the one belong identically and in the same degree to the other; and (b) that unlikes, i.e. differents, are by nature such as to act and suffer action reciprocally. For even when the smaller fire is destroyed by the greater, it suffers this effect (they say) owing to its contrariety since the great is contrary to the small. But (ii) Democritus dissented from all the other thinkers and maintained a theory peculiar to himself. He asserts that agent and patient are identical, i.e. like. It is not possible (he says) that others, i.e. differents, should suffer action from one another: on the contrary, even if two things, being others, do act in some way on one another, this happens to them not qua others but qua possessing an identical property.
Such, then, are the traditional theories, and it looks as if the statements of their advocates were in manifest conflict. But the reason of this conflict is that each group is in fact stating a part, whereas they ought to have taken a comprehensive view of the subject as a whole. For (i) if A and B are like-absolutely and in all respects without difference from one another --it is reasonable to infer that neither is in any way affected by the other. Why, indeed, should either of them tend to act any more than the other? Moreover, if like can be affected by like, a thing can also be affected by itself: and yet if that were so-if like tended in fact to act qua like-there would be nothing indestructible or immovable, for everything would move itself. And (ii) the same consequence follows if A and B are absolutely other, i.e. in no respect identical. Whiteness could not be affected in any way by line nor line by whiteness--except perhaps coincidentally, viz. if the line happened to be white or black: for unless two things either are, or are composed of, contraries, neither drives the other out of its natural condition. But (iii) since only those things which either involve a contrariety or are contraries--and not any things selected at random-are such as to suffer action and to act, agent and patient must be like (i.e. identical) in kind and yet unlike (i.e. contrary) in species. (For it is a law of nature that body is affected by body, flavour by flavour, colour by colour, and so in general what belongs to any kind by a member of the same kind-the reason being that contraries are in every case within a single identical kind, and it is contraries which reciprocally act and suffer action.) Hence agent and patient must be in one sense identical, but in another sense other than (i.e. unlike) one another. And since (a) patient and agent are generically identical (i.e. like) but specifically unlike, while (b) it is contraries that exhibit this character: it is clear that contraries and their intermediates are such as to suffer action and to act reciprocally-for indeed it is these that constitute the entire sphere of passing-away and coming-to-be.
We can now understand why fire heats and the cold thing cools, and in general why the active thing assimilates to itself the patient. For agent and patient are contrary to one another, and coming-to-be is a process into the contrary: hence the patient must change into the agent, since it is only thus that coming-to be will be a process into the contrary. And, again, it is intelligible that the advocates of both views, although their theories are not the same, are yet in contact with the nature of the facts. For sometimes we speak of the substratum as suffering action (e.g. of the man as being healed, being warmed and chilled, and similarly in all the other cases), but at other times we say what is cold is being warmed, what is sick is being healed: and in both these ways of speaking we express the truth, since in one sense it is the matter, while in another sense it is the contrary, which suffers action. (We make the same distinction in speaking of the agent: for sometimes we say that the man, but at other times that what is hot, produces heat.) Now the one group of thinkers supposed that agent and patient must possess something identical, because they fastened their attention on the substratum: while the other group maintained the opposite because their attention was concentrated on the contraries. We must conceive the same account to hold of action and passion as that which is true of being moved and imparting motion. For the mover, like the agent, has two meanings. Both (a) that which contains the originative source of the motion is thought to impart motion (for the originative source is first amongst the causes), and also (b) that which is last, i.e. immediately next to the moved thing and to the coming-to-be. A similar distinction holds also of the agent: for we speak not only (a) of the doctor, but also (b) of the wine, as healing. Now, in motion, there is nothing to prevent the firs; mover being unmoved (indeed, as regards some first movers this is actually necessary) although the last mover always imparts motion by being itself moved: and, in action, there is nothing to prevent the first agent being unaffected, while the last agent only acts by suffering action itself. For agent and patient have not the same matter, agent acts without being affected: thus the art of healing produces health without itself being acted upon in any way by that which is being healed. But (b) the food, in acting, is itself in some way acted upon: for, in acting, it is simultaneously heated or cooled or otherwise affected. Now the art of healing corresponds to an originative source, while the food corresponds to the last (i.e. continuous) mover.
Those active powers, then, whose forms are not embodied in matter, are unaffected: but those whose forms are in matter are such as to be affected in acting. For we maintain that one and the same matter is equally, so to say, the basis of either of the two opposed things-being as it were a kind; and that that which can he hot must be made hot, provided the heating agent is there, i.e. comes near. Hence (as we have said) some of the active powers are unaffected while others are such as to be affected; and what holds of motion is true also of the active powers. For as in motion the first mover is unmoved, so among the active powers the first agent is unaffected.
The active power is a cause in the sense of that from which the process originates: but the end, for the sake of which it takes place, is not active. (That is why health is not active, except metaphorically.) For when the agent is there, the patient he-comes something: but when states are there, the patient no longer becomes but already is-and forms (i.e. lends) are a kind of state. As to the matter, it (qua matter) is passive. Now fire contains the hot embodied in matter: but a hot separate from matter (if such a thing existed) could not suffer any action. Perhaps, indeed, it is impossible that the hot should exist in separation from matter: but if there are any entities thus separable, what we are saying would be true of them.
We have thus explained what action and passion are, what things exhibit them, why they do so, and in what manner. We must go on to discuss how it is possible for action and passion to take place.