Objection 1. It would seem that an angel is composed of matter and form. For everything which is contained under any genus is composed of the genus, and of the difference which added to the genus makes the species. But the genus comes from the matter, and the difference from the form (Metaph. xiii, text 6). Therefore everything which is in a genus is composed of matter and form. But an angel is in the genus of substance. Therefore he is composed of matter and form.
Objection 2. Further, wherever the properties of matter exist, there is matter. Now the properties of matter are to receive and to substand; whence Boethius says (De Trin.) that "a simple form cannot be a subject": and the above properties are found in the angel. Therefore an angel is composed of matter and form.
Objection 3. Further, form is act. So what is form only is pure act. But an angel is not pure act, for this belongs to God alone. Therefore an angel is not form only, but has a form in matter.
Objection 4. Further, form is properly limited and perfected by matter. So the form which is not in matter is an infinite form. But the form of an angel is not infinite, for every creature is finite. Therefore the form of an angel is in matter.
On the contrary, Dionysius says (Div. Nom. iv): "The first creatures are understood to be as immaterial as they are incorporeal."
I answer that, Some assert that the angels are composed of matter and form; which opinion Avicebron endeavored to establish in his book of the Fount of Life. For he supposes that whatever things are distinguished by the intellect are really distinct. Now as regards incorporeal substance, the intellect apprehends that which distinguishes it from corporeal substance, and that which it has in common with it. Hence he concludes that what distinguishes incorporeal from corporeal substance is a kind of form to it, and whatever is subject to this distinguishing form, as it were something common, is its matter. Therefore, he asserts the universal matter of spiritual and corporeal things is the same; so that it must be understood that the form of incorporeal substance is impressed in the matter of spiritual things, in the same way as the form of quantity is impressed in the matter of corporeal things.
But one glance is enough to show that there cannot be one matter of spiritual and of corporeal things. For it is not possible that a spiritual and a corporeal form should be received into the same part of matter, otherwise one and the same thing would be corporeal and spiritual. Hence it would follow that one part of matter receives the corporeal form, and another receives the spiritual form. Matter, however, is not divisible into parts except as regarded under quantity; and without quantity substance is indivisible, as Aristotle says (Phys. i, text 15). Therefore it would follow that the matter of spiritual things is subject to quantity; which cannot be. Therefore it is impossible that corporeal and spiritual things should have the same matter.
It is, further, impossible for an intellectual substance to have any kind of matter. For the operation belonging to anything is according to the mode of its substance. Now to understand is an altogether immaterial operation, as appears from its object, whence any act receives its species and nature. For a thing is understood according to its degree of immateriality; because forms that exist in matter are individual forms which the intellect cannot apprehend as such. Hence it must be that every intellectual substance is altogether immaterial.
But things distinguished by the intellect are not necessarily distinguished in reality; because the intellect does not apprehend things according to their mode, but according to its own mode. Hence material things which are below our intellect exist in our intellect in a simpler mode than they exist in themselves. Angelic substances, on the other hand, are above our intellect; and hence our intellect cannot attain to apprehend them, as they are in themselves, but by its own mode, according as it apprehends composite things; and in this way also it apprehends God (3).
Reply to Objection 1. It is difference which constitutes the species. Now everything is constituted in a species according as it is determined to some special grade of being because "the species of things are like numbers," which differ by addition and subtraction of unity, as the Philosopher says (Metaph. viii, text 10). But in material things there is one thing which determines to a special grade, and that is the form; and another thing which is determined, and this is the matter; and hence from the latter the "genus" is derived, and from the former the "difference." Whereas in immaterial things there is no separate determinator and thing determined; each thing by its own self holds a determinate grade in being; and therefore in them "genus" and "difference" are not derived from different things, but from one and the same. Nevertheless, this differs in our mode of conception; for, inasmuch as our intellect considers it as indeterminate, it derives the idea of their "genus"; and inasmuch as it considers it determinately, it derives the idea of their "difference."
Reply to Objection 2. This reason is given in the book on the Fount of Life, and it would be cogent, supposing that the receptive mode of the intellect and of matter were the same. But this is clearly false. For matter receives the form, that thereby it may be constituted in some species, either of air, or of fire, or of something else. But the intellect does not receive the form in the same way; otherwise the opinion of Empedocles (De Anima i, 5, text 26) would be true, to the effect that we know earth by earth, and fire by fire. But the intelligible form is in the intellect according to the very nature of a form; for as such is it so known by the intellect. Hence such a way of receiving is not that of matter, but of an immaterial substance.
Reply to Objection 3. Although there is no composition of matter and form in an angel, yet there is act and potentiality. And this can be made evident if we consider the nature of material things which contain a twofold composition. The first is that of form and matter, whereby the nature is constituted. Such a composite nature is not its own existence but existence is its act. Hence the nature itself is related to its own existence as potentiality to act. Therefore if there be no matter, and supposing that the form itself subsists without matter, there nevertheless still remains the relation of the form to its very existence, as of potentiality to act. And such a kind of composition is understood to be in the angels; and this is what some say, that an angel is composed of, "whereby he is," and "what is," or "existence," and "what is," as Boethius says. For "what is," is the form itself subsisting; and the existence itself is whereby the substance is; as the running is whereby the runner runs. But in God "existence" and "what is" are not different as was explained above (Question 3, Article 4). Hence God alone is pure act.
Reply to Objection 4. Every creature is simply finite, inasmuch as its existence is not absolutely subsisting, but is limited to some nature to which it belongs. But there is nothing against a creature being considered relatively infinite. Material creatures are infinite on the part of matter, but finite in their form, which is limited by the matter which receives it. But immaterial created substances are finite in their being; whereas they are infinite in the sense that their forms are not received in anything else; as if we were to say, for example, that whiteness existing separate is infinite as regards the nature of whiteness, forasmuch as it is not contracted to any one subject; while its "being" is finite as determined to some one special nature.
Whence it is said (De Causis, prop. 16) that "intelligence is finite from above," as receiving its being from above itself, and is "infinite from below," as not received in any matter.
Source: Summa Theologica (New Advent)