On Generation and corruption
must be reciprocal. The reason of this belief is that 'movers' which
belong to the same kind as the 'moved' impart motion by being moved.
Hence if anything imparts motion without itself being moved, it may
touch the 'moved' and yet itself be touched by nothing-for we say
sometimes that the man who grieves us 'touches' us, but not that we
The account just given may serve to distinguish and define the
'contact' which occurs in the things of Nature.
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