This is true, unless, perhaps, there is something whose quiddity is its very act of existing. This thing would have to be unique and primary, since it would be impossible for anything to be multiplied except by the addition of some difference, as the nature genus is multiplied into species; or by a form being received in diverse matters, as the nature species is multiplied in dfferent individuals; or by one being absolute, and the other being received in something. For example, if there were a certain "separated" heat it would be distinct, in virtue of its very separation, from the heat which is not separated.

If, however, something is posited which is simply its own act of existing, such that it would be subsistent existence itself, this existence cannot recieve the addition of a difference, because then it would not be simply an act of existing, but an act of existing plus this certain form. Even less would it receive the addition of matter, because then it would not be subsistent existence but material existence. Hence, there remains only one such thing that is its own act of existing. Accordingly, in anything other than it, the act of existing must necessarily be other than its quiddity or nature or form. Hence among the intelligences (angels), their acts of existing must be other that their forms. Therefore, it is said that intelligences are (composed as) forms and acts of existing.

Whatever belongs to something is either caused by the principles of its nature, like risibility in man, or accrues to it from some extrinsic principle, like the light in the air which is caused by the sun. It is impossible that the act of existing itself be caused by the form or quiddity -- and by "caused" I mean as by an efficient cause -- for then something would be the cause of itself and produce itself in existence which is impossible. It is therefore necessary that everything whose act of existing is other than its nature have its act of existing from another. And because everything which exists through another is reduced to that which exists through itself, as to a first cause, there must be something which causes all things to exist, inasmuch as it is subsistent existence alone. Otherwise we would proceed to infinity in causes, since everything which is not a subsistent act of existing has a cause for its act of existing, as we have just said. It is evident, therefore, that an intelligence is form and an act of existing, and that it has its act of existing from the First Being which is (simply) existence only; and this it the First Cause, God.

Adapted from Selected Writings of St. Thomas Aquinas, Robert P. Goodwin, trans., (Indianapolis: Bobbs-Merrill, 1965).

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