Objection 1. It would seem that the formless matter of all corporeal things is the same. For Augustine says (Confess. xii, 12): "I find two things Thou hast made, one formed, the other formless," and he says that the latter was the earth invisible and shapeless, whereby, he says, the matter of all corporeal things is designated. Therefore the matter of all corporeal things is the same.
Objection 2. Further, the Philosopher says (Metaph. v, text. 10): "Things that are one in genus are one in matter." But all corporeal things are in the same genus of body. Therefore the matter of all bodies is the same.
Objection 3. Further, different acts befit different potentialities, and the same act befits the same potentiality. But all bodies have the same form, corporeity. Therefore all bodies have the same matter.
Objection 4. Further, matter, considered in itself, is only in potentiality. But distinction is due to form. Therefore matter considered in itself is the same in all corporeal things.
On the contrary, Things of which the matter is the same are mutually interchangeable and mutually active or passive, as is said (De Gener. i, text. 50). But heavenly and earthly bodies do not act upon each other mutually. Therefore their matter is not the same.
I answer that, On this question the opinions of philosophers have differed. Plato and all who preceded Aristotle held that all bodies are of the nature of the four elements. Hence because the four elements have one common matter, as their mutual generation and corruption prove, it followed that the matter of all bodies is the same. But the fact of the incorruptibility of some bodies was ascribed by Plato, not to the condition of matter, but to the will of the artificer, God, Whom he represents as saying to the heavenly bodies: "By your own nature you are subject to dissolution, but by My will you are indissoluble, for My will is more powerful than the link that binds you together." But this theory Aristotle (De Caelo i, text. 5) disproves by the natural movements of bodies. For since, he says, the heavenly bodies have a natural movement, different from that of the elements, it follows that they have a different nature from them. For movement in a circle, which is proper to the heavenly bodies, is not by contraries, whereas the movements of the elements are mutually opposite, one tending upwards, another downwards: so, therefore, the heavenly body is without contrariety, whereas the elemental bodies have contrariety in their nature. And as generation and corruption are from contraries, it follows that, whereas the elements are corruptible, the heavenly bodies are incorruptible. But in spite of this difference of natural corruption and incorruption, Avicebron taught unity of matter in all bodies, arguing from their unity of form. And, indeed, if corporeity were one form in itself, on which the other forms that distinguish bodies from each other supervene, this argument would necessarily be true; for this form of corporeity would inhere in matter immutably and so far all bodies would be incorruptible. But corruption would then be merely accidental through the disappearance of successive forms--that is to say, it would be corruption, not pure and simple, but partial, since a being in act would subsist under the transient form. Thus the ancient natural philosophers taught that the substratum of bodies was some actual being, such as air or fire. But supposing that no form exists in corruptible bodies which remains subsisting beneath generation and corruption, it follows necessarily that the matter of corruptible and incorruptible bodies is not the same. For matter, as it is in itself, is in potentiality to form.
Considered in itself, then, it is in potentiality in respect to all those forms to which it is common, and in receiving any one form it is in act only as regards that form. Hence it remains in potentiality to all other forms. And this is the case even where some forms are more perfect than others, and contain these others virtually in themselves. For potentiality in itself is indifferent with respect to perfection and imperfection, so that under an imperfect form it is in potentiality to a perfect form, and "vice versa." Matter, therefore, whilst existing under the form of an incorruptible body, would be in potentiality to the form of a corruptible body; and as it does not actually possess the latter, it has both form and the privation of form; for want of a form in that which is in potentiality thereto is privation. But this condition implies corruptibility. It is therefore impossible that bodies by nature corruptible, and those by nature incorruptible, should possess the same matter.
Neither can we say, as Averroes [De Substantia Orbis ii.] imagines, that a heavenly body itself is the matter of the heaven--beings in potentiality with regard to place, though not to being, and that its form is a separate substance united to it as its motive force. For it is impossible to suppose any being in act, unless in its totality it be act and form, or be something which has act or form. Setting aside, then, in thought, the separate substance stated to be endowed with motive power, if the heavenly body is not something having form--that is, something composed of a form and the subject of that form--it follows that in its totality it is form and act. But every such thing is something actually understood, which the heavenly bodies are not, being sensible. It follows, then, that the matter of the heavenly bodies, considered in itself, is in potentiality to that form alone which it actually possesses. Nor does it concern the point at issue to inquire whether this is a soul or any other thing. Hence this form perfects this matter in such a way that there remains in it no potentiality with respect to being, but only to place, as Aristotle [De Coelo i, text. 20 says. So, then, the matter of the heavenly bodies and of the elements is not the same, except by analogy, in so far as they agree in the character of potentiality.
Reply to Objection 1. Augustine follows in this the opinion of Plato, who does not admit a fifth essence. Or we may say that formless matter is one with the unity of order, as all bodies are one in the order of corporeal creatures.
Reply to Objection 2. If genus is taken in a physical sense, corruptible and incorruptible things are not in the same genus, on account of their different modes of potentiality, as is said in Metaph. x, text. 26. Logically considered, however, there is but one genus of all bodies, since they are all included in the one notion of corporeity.
Reply to Objection 3. The form of corporeity is not one and the same in all bodies, being no other than the various forms by which bodies are distinguished, as stated above.
Reply to Objection 4. As potentiality is directed towards act, potential beings are differentiated by their different acts, as sight is by color, hearing by sound. Therefore for this reason the matter of the celestial bodies is different from that of the elemental, because the matter of the celestial is not in potentiality to an elemental form.
Source: Summa Theologica (New Advent)