But it is logically necessary to presuppose, prior to the potentiality of becoming fire or any other particular and definite kind of thing, a potentiality for becoming at all, i.e. a bare potentiality. Modern nuclear physics tells us that the matter of every thing can ultimately and theoretically be converted into energy, and energy can similarly be converted into matter. Nevertheless, there still seems to be some principle that underlies each of these two ways of being physical but cannot exist without being one or the other, matter or energy. Thus, it seems that we still have a theoretical need for the notion of prime matter. Formal cause Each and every individual thing is composed not only of matter, but also of form.
Form is the principle of determination which accounts for the thing being the kind of thing it is.
As matter is the principle of potentiality, of the ability to become other than it is, form is the principle of actuality, of the thing being the sort of thing it is. In natural things, the substantial form is specifically the same for all members of the same species. That is, the substantial form of a thing accounts for it being the sort of thing it is, for belonging to the species to which it does belong. Thus, substantial form, considered apart from any matter in which it resides, is universal. Although form connotes shape or configuration, Aristotle means by form something more.