De principiis naturae

Note that something can be although it is not, while something else is. What can be is said to be in potency; what already is is said to be in act. But being is twofold: namely, the essential or substantial being of a thing, as for a man to be — and this is being simply; and there is another, accidental being, as for a man to be white — and this is to be something. Toward each kind of being there is something in potency: for something is in potency to be a man, as the seed and the menstrual blood, and something is in potency to be white, as a man. Both what is in potency to substantial being and what is in potency to accidental being can be called matter — as the seed of a man, and a man [as matter] of whiteness; but they differ in this: the matter which is in potency to substantial being is called the matter out of which, while what is in potency to accidental being is called the matter in which. Likewise, properly speaking, what is in potency to accidental being is called a subject, while what is in potency to substantial being is properly called matter. And a sign that what is in potency to accidental being is called a subject is that accidents are said to be in a subject, whereas a substantial form is not said to be in a subject. And accordingly matter differs from a subject, since a subject does not have being from what comes to it but has complete being of itself — as a man does not have being from whiteness — while matter has being from what comes to it, since of itself it has incomplete being. Hence, speaking simply, form gives being to matter, but the subject gives it to the accident, although sometimes one is taken for the other, namely matter for subject and conversely. But just as everything which is in potency can be called matter, so everything from which a thing has being, whatever being it be — substantial or accidental — can be called form: as a man, being potentially white, becomes actually white through whiteness, and seed, being potentially a man, becomes actually a man through the soul. And since form makes a thing be in act, form is therefore said to be act; and what makes a thing actually be substantially is a substantial form, while what makes it actually be accidentally is called an accidental form. And since generation is a motion toward form, a twofold form is matched by a twofold generation: to a substantial form corresponds generation simply, to an accidental form generation in a certain respect. For when a substantial form is introduced, a thing is said to come to be simply; but when an accidental form is introduced, it is not said to come to be simply, but to come to be this — as when a man becomes white, we do not say simply that the man comes to be or is generated, but that he comes to be or is generated white. And to this twofold generation corresponds a twofold corruption: namely, simply and in a certain respect. Generation and corruption simply, however, occur only in the genus of substance; generation and corruption in a certain respect occur in the other genera. And since generation is a certain change from non-being — that is, from a being — to being — that is, to a being — while corruption conversely must be from being to non-being, generation does not come from just any non-being, but from the non-being which is a being in potency — as a statue comes from bronze, to which the statue stands in potency, not in act. For generation, then, three things are required: namely, being in potency, which is matter; and non-being in act, which is privation; and that through which it becomes actual, namely form. As when a statue is made from bronze: the bronze, which is in potency to the form of the statue, is the matter; what is unshaped or unarranged is called privation; the shape from which it is called a statue is the form — not a substantial one, since bronze has actual being before the coming of the form or figure, and its being does not depend on that figure — but an accidental form. For all artificial forms are accidental: art operates only upon what has already been constituted in perfect being by nature.