[1069b] [1] The material form of the substance

the last belongs to some other science, if there is no principle common to all three.
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 contrary —then there must be something underlying which changes into the opposite contrary; for the contraries do not change.

Further, something persists, whereas the contrary does not persist. Therefore besides the contraries there is some third thing,  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. It must be  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. And this is the “one” of Anaxagoras; for his “all things were together,” and the “mixture” of Empedocles and Anaximander and the doctrine of Democritus would be better expressed as “all things were together potentially, but not actually.” Hence these thinkers must have had some conception of matter. All things which change have matter, but different things have different kinds; and of eternal things such as are not generable but are movable by locomotion have matter; matter, however, which admits not of generation, but of motion from one place to another.

One might raise the question from what sort of “not-being” generation takes place; for not-being has three senses. If a thing exists through a potentiality, nevertheless it is not through a potentiality for any chance thing; different things are derived from different things.Nor is it satisfactory to say that “all things were together,” for they differ in their matter, since otherwise why did they become an infinity and not one? For Mind is one; so that if matter is also one, only that could have come to be in actuality whose matter existed potentially. The causes and principles, then, are three; two being the pair of contraries, of which one is the formula or form and the other the privation, and the third being the matter.

We must next observe that neither matter nor form (I mean in the proximate sense) is generated. All change is of some subject by some agent into some object.