On Generation and corruption
Nevertheless, since they are four, each of them is characterized par
excellence a single quality: Earth by dry rather than by cold, Water
by cold rather than by moist, Air by moist rather than by hot, and
Fire by hot rather than by dry.
It has been established before' that the coming-to-be of the
'simple' bodies is reciprocal. At the same time, it is manifest,
even on the evidence of perception, that they do come-to-be: for
otherwise there would not have been 'alteration, since 'alteration' is
change in respect to the qualities of the objects of touch.
Consequently, we must explain (i) what is the manner of their
reciprocal transformation, and (ii) whether every one of them can come
to-be out of every one-or whether some can do so, but not others.
Now it is evident that all of them are by nature such as to change
into one another: for coming-to-be is a change into contraries and out
of contraries, and the 'elements' all involve a contrariety in their
mutual relations because their distinctive qualities are contrary. For
in some of them both qualities are contrary-e.g. in Fire and Water,
the first of these being dry and hot, and the second moist and cold:
while in others one of the qualities (though only one) is
contrary-e.g. in Air and Water, the first being moist and hot, and the
second moist and cold. It is evident, therefore, if we consider them
in general, that every one is by nature such as to come-to-be out of
every one: and when we come to consider them severally, it is not
difficult to see the manner in which their transformation is effected.
For, though all will result from all, both the speed and the
facility of their conversion will differ in degree.
Thus (i) the process of conversion will be quick between those which
have interchangeable 'complementary factors', but slow between those
which have none. The reason is that it is easier for a single thing to