On Longevity And Shortness Of Life   

place where what is corruptible becomes incorruptible, as fire does in

the upper regions where it meets with no opposite. Opposites destroy

each other, and hence accidentally, by their destruction, whatsoever

is attributed to them is destroyed. But no opposite in a real

substance is accidentally destroyed, because real substance is not

predicated of any subject. Hence a thing which has no opposite, or

which is situated where it has no opposite, cannot be destroyed. For

what will that be which can destroy it, if destruction comes only

through contraries, but no contrary to it exists either absolutely

or in the particular place where it is? But perhaps this is in one

sense true, in another sense not true, for it is impossible that

anything containing matter should not have in any sense an opposite.

Heat and straightness can be present in every part of a thing, but

it is impossible that the thing should be nothing but hot or white

or straight; for, if that were so, attributes would have an

independent existence. Hence if, in all cases, whenever the active and

the passive exist together, the one acts and the other is acted on, it

is impossible that no change should occur. Further, this is so if a

waste product is an opposite, and waste must always be produced; for

opposition is always the source of change, and refuse is what

remains of the previous opposite. But, after expelling everything of a

nature actually opposed, would an object in this case also be

imperishable? No, it would be destroyed by the environment.

If then that is so, what we have said sufficiently accounts for

the change; but, if not, we must assume that something of actually

opposite character is in the changing object, and refuse is produced.

Hence accidentally a lesser flame is consumed by a greater one,

for the nutriment, to wit the smoke, which the former takes a long

period to expend, is used up by the big flame quickly.

Hence [too] all things are at all times in a state of transition and

are coming into being and passing away. The environment acts on them

either favourably or antagonistically, and, owing to this, things that