On the Principles of Nature (De principiis naturae) by Thomas Aquinas Notice carefully that some thing can be although it is not, whereas another thing truly is.That which can be is called being in potency; that which already is, is called being in act.But there are two kinds of being: namely essential being or the substantial being of the thing (as, for example, to be a man)and this is to be absolutely.The other kind of being is accidental being (as, for example, to be a white man), and this is to be after-a-fashion or in some qualified way.

To each of these kinds of being (i.e.substantial being and accidental being) there is something which is in potency.For there is something which is in potency to be a man, namely, the sperm and the menstrual blood; and there is also something which is in potency to being white, namely, a man.And that which is in potency to substantial being just as much as that which is in potency to accidental being can be called matter; as, for example, the sperm is with respect to the man and the man, in respect to whiteness.But there is a difference.The matter which is in potency to substantial being is called the matter 'out of which' whereas the matter which is in potency to accidental being is called the matter 'in which'.Again, in a strict way of speaking, that which is in potency to substantial being is called prime matter, however, that which is in potency to accidental being is called the subject: This is why it is said that accidents are in a subject--and also why it is not said that substantial form is in a subject.And it is according to this notion that 'matter' differs from 'subject' because the subject does not have its being from what comes to it but has in itself complete being (as, for example, a man does not derive his being from his whiteness).But matter, on the other hand, does derive its being from what comes to it, since, of itself, it has incomplete being.Therefore, strictly speaking, (substantial) form gives being to matter, whereas accidents do not give being to a subject, but rather, the subject to the accidents.

Sometimes, however (in popular terminology), one term is used for the other, that is, 'matter' for 'subject' and vice-versa.

Again, just as everything which is in potency can be called 'matter' in the same way everything by which something has being (either substantially or accidentally), can be called 'form'; for example, in man since he is potentially white, he becomes actually white through whiteness, and the sperm, since it is potentially man becomes actually man through the soul.And because it is the form which makes something to be in act, accordingly the form is called 'act'.Moreover, that which brings about substantial being in act is called 'substantial form', and that which brings about accidental being in act is called 'accidental form'.

Now since generation (coming-to-be) is a movement towards form, there are two kinds of generation which correspond to these two kinds of forms: absolute coming-to-be corresponds to substantial form-relative coming-to-be (coming-to-be-in-a-certain-way), corresponds to accidental form.For whenever substantial form is introduced, we say that something absolutely becomes (i.e.without qualification)--for example, a man becomes a man or a man comes-to-be.However, when accidental form is introduced, we do not say that something becomes absolutely--but rather that it becomes this--for example, when a man becomes white we do not say that he becomes a man or that a man has come-to-be but rather that the man becomes white or comes-to-be white.And there is a two-fold notion of corruption (passing away) which is opposed to this two-fold notion of generation (coming-to-be)--namely, absolute passing-away and relative passing away.Absolute generation (coming-to-be) and absolute corruption (passing away) exist only within the genus of substance.On the other hand relative generation and relative corruption (coming-to-be-and passing-away-in some way or other) are found in all of the other genera.And because generation is a kind of movement from non-being to being and, conversely, corruption from being to non-being, it is not from just any kind of non-being that generation comes but from non-being which is being in potency--just as, for example, the statue comes from bronze which is statue in potency, not in act.

And, therefore, in order that generation (coming-to-be) come about, three things are necessary:

namely, a potential something which is the matter, and not-being in act which is the privation, and that through which it becomes in act, namely, the form.Let us take an example: when a statue-is made from bronze, the bronze which is in potency for the form of the statue is 'the matter';the privation is the shapelessness or the lack of the form (of statue in the molten bronze); the shape by which we call it a statue is the form.

The form of statue, however, is not the substantial form, because the bronze before the coming-to-be of this form (of statue) has being in act, and its being does not depend upon this shape (of statue) which is an accidental form.All artificial forms are accidental forms.For art only operates upon those things already constituted in being by nature.