Back to basics this week, with some important links which do a better job summarizing these topics than I can do easily.
1 Now composition of matter and form is not of the same nature as composition of substance and being, although both result from potentiality and act.
Notes About matter and form we have done much already. Substance: “signifies being as existing in and by itself, and serving as a subject or basis for accidents and accidental changes.” A piece of wood can be lying on its side, upright, painted white or plain, yet it is still a piece of wood.
It is necessary, therefore, to recognize in each thing certain secondary realities (see ACCIDENT) and also a permanent fundamentum which continues to exist notwithstanding the superficial changes, which serves as a basis or support for the secondary realities — what, in a word, we term the substance. Its fundamental characteristic is to be in itself and by itself, and not in another subject as accidents are.
2 First, because matter is not the very substance of a thing, else it would follow that all forms are accidental, as the early natural philosophers maintained; but matter is part of the substance.
Notes There has to be permanent fixtures of form; there is something that makes all chairs chairs. Incidentally, see also nominalism.
3 Secondly, because being itself is the proper act, not of matter, but of the whole substance: for being is the act of that whereof we can say that it is. Now being is said, not of matter but of the whole. Therefore we cannot say of matter that it is, but the substance itself is that which is.
4 Thirdly, because neither is the form being itself, but they are related as things in an order: because form is compared to being as light to enlightening, or whiteness to being white.
5 Also, because being itself is compared as act even to the very form. For in things composed of matter and form, the form is said to be the principle of being, for the reason that it is the complement of substance, whose act being is: even as transparency is to the air the principle of being lightsome, in that it makes the air the proper subject of light.
6 Wherefore in things composed of matter and form, neither matter nor form, nor even being itself, can be described as that which is. Yet the form can be described as that whereby it is, or asmuch as it is the principle of being: but the whole substance is what is; and being is that whereby the substance is called a being.
Notes In being were everything, then there’d be no differentiating anything that exists.
7 But in intellectual substances, which are not composed of matter and form, as shown above, and wherein the form itself is a subsistent substance, the form is what is, and being is the act whereby it is.
8 Consequently in them there is but one composition of act and potentiality, a composition namely of substance and being, which by some is said to be of what is and being, or of what is and whereby it is.
9 On the other hand in things composed of matter and form there is a twofold composition of act and potentiality: the first, of the substance itself which is composed of matter and form; the second, of the already composite substance, and being, which composition can also be said to be of what is and being, or of what is and whereby it is.
Notes So much is straightforward. The real magic happens next and last, because act and potentiality apply to things in the material world, but also to created things in the spiritual world, like our intellects and angels.
10 It is therefore evident that composition of act and potentiality covers more ground than composition of form and matter. Wherefore matter and form divide a natural substance, while potentiality and act divide being in general. For this reason whatever is consequent upon potentiality and act, as such, is common to created substances whether material or immaterial; for instance to receive and to be received, to perfect and to be perfected. Whereas whatsoever things are proper to matter and form, as such, for instance to be generated and to be corrupted and so forth, are proper to material substances, and are nowise applicable to created immaterial substances.
Notes Which boosts the point of our intellects being subsistent.