Book 12
5
Sensible substance is liable to change. Now if change proceeds from opposites or intermediates—not however from all opposites (for speech is not white), but only from the contraryCf. Aristot. Met. 10.7.—then there must be something underlying which changes into the opposite contrary; for the contrariesi.e., contrary qualities. Cf. Aristot. Met. 8.5.1. do not change.
1
Further, something persists, whereas the contrary does not persist. Therefore besides the contraries there is some third thing, the matter . Now if change is of four kinds, in respect either of substance or of quality or of quantity or of place, and if change of substance is generation or destruction in the simple sense, and change of quantity is increase or decrease, and change of affection is alteration, and change of place is locomotion, then changes must be in each case into the corresponding contrary state.
2
It must be the matter, then, which admits of both contraries, that changes. And since that which is is twofold, everything changes from that which is potentially to that which is actually; e.g. from potentially white to actually white. The same applies to increase and decrease. Hence not only may there be generation accidentally from that which is not, but also everything is generated from that which is, but is potentially and is not actually.
