Having just argued that angels are pure forms, and have no matter whatsoever (not even “spiritual matter” which Franciscan theologians, like St. Bonaventure, posited), Aquinas goes on to explain that angels nevertheless have some composition. Thus, they are not altogether simple beings. This is important for him to explain because traditionally only God is completely simple, i.e. the Being without any composition at all. For the Christian theologians of the Middle Ages, God is properly understood to be simple and the only simple being, because everything that is composite depends on what it is composed of. God does not depend on anything, but everything other than Him depends on Him.
On Being and Essence, Ch. 4 (excerpt)
Although substances of this kind (separate intelligences or angels) are simple forms without matter, nonetheless they are not in every way simple as pure acts are. They do have an admixture of potency, which is evident in the following way. Whatever is extraneous to the concept of an essence or quiddity comes to it from beyond itself, and forms a composition with the essence since no essence can be understood without those things which are its parts. On the other hand, every essence or quiddity can be understood without its act of existing being understood. I can understand what a man or phoenix is, and yet not know whether or not it exists in reality. Therefore, it is evident that the act of existing is other than essence or quiddity.
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 different 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 receive 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).