On Generation and corruption   

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 whiseness-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