The second interpretation of the words _materia prima_ is that given by S. Thomas, when he considers the first matter as “matter without form,” and as a mere potency of being. “The matter,” he says, “exists sometimes under one form, and at other times under another; but it can never exist isolated--that is, by itself--because, as it does not involve in its ratio any form, it cannot be in act (for the form is the only source of actuality), but can merely be in potency. And therefore, _nothing which is in act can be called first matter_.”[17] From these words it is evident that S. Thomas considers the first matter as matter without form; for, had it a form, it would be in act, and would cease to be called “first” matter. In another place he says: “Since the matter is a pure potency, it is _one_, not through any one form actuating it, but _by the exclusion of all forms_.”[18] And again: “The accidental form supervenes to a subject already pre-existent in act; the substantial form, on the contrary, does not supervene to a subject already pre-existent in act, but _to something which is merely in potency to exist_, viz., _to the first matter_.”[19] And again: “The true nature of matter is _to have no form whatever in act_, but to be in potency with regard to any of them.”[20] And again: “The first matter is a pure potency, just as God is a pure act.”[21] From these passages, and from many others that might be found in S. Thomas’ works, it is manifest that the holy doctor, in his metaphysical speculations, considers the first matter as matter without a form. In this he faithfully follows Aristotle’s doctrine. For the Greek philosopher explicitly teaches that “as the metal is to the statue, or the wood to the bedstead, or any other unformed material to the thing which can be formed with it, so is the matter to the substance and to the being”;[22] that is, as the metal has not yet the form of a statue, so the first matter still lacks the substantial form, and consequently is a _pure_ potency of being. Nevertheless, the Angelic Doctor does not always abide by this old and genuine notion of the first matter. When treating of generation and corruption, or engaged in other physical questions, he freely assumes that the first matter is something actually lying under a substantial form, and therefore that it is a _real_ potency in the order of nature, and not a mere result of intellectual abstraction. Thus he concedes that “the first matter exists in all bodies,”[23] that “the first matter must have been created by God under a substantial form,”[24] and that “the first matter remains in act, after it has lost a certain form, owing to the fact that it is actuated by another form.”[25] In these passages and in many others the first matter is evidently considered as matter under a form.