The former two kinds of
substance are the subject of physics (for they imply movement); but
the third kind belongs to another science, if there is no principle
common to it and to the other kinds.
2
Sensible substance is changeable. Now if change proceeds from
opposites or from intermediates, and not from all opposites (for the
voice is not-white, (but it does not therefore change to white)),
but from the contrary, there must be something underlying which
changes into the contrary state; for the contraries do not change.
Further, something persists, but the contrary does not persist;
there is, then, some third thing besides the contraries, viz. the
matter. Now since changes are of four kinds-either in respect of the
'what' or of the quality or of the quantity or of the place, and
change in respect of 'thisness' is simple generation and
destruction, and change in quantity is increase and diminution, and
change in respect of an affection is alteration, and change of place
is motion, changes will be from given states into those contrary to
them in these several respects. The matter, then, which changes must
be capable of both states. And since that which 'is' has two senses,
we must say that everything changes from that which is potentially
to that which is actually, e.g. from potentially white to actually
white, and similarly in the case of increase and diminution. Therefore
not only can a thing come to be, incidentally, out of that which is
not, but also all things come to be out of that which is, but is
potentially, and is not actually.
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