Objection 1. It seems that bread cannot be converted into the body of Christ. For conversion is a kind of change. But in every change there must be some subject, which from being previously in potentiality is now in act. because as is said in Phys. iii: "motion is the act of a thing existing in potentiality." But no subject can be assigned for the substance of the bread and of the body of Christ, because it is of the very nature of substance for it "not to be in a subject," as it is said in Praedic. iii. Therefore it is not possible for the whole substance of the bread to be converted into the body of Christ.
Objection 2. Further, the form of the thing into which another is converted, begins anew to inhere in the matter of the thing converted into it: as when air is changed into fire not already existing, the form of fire begins anew to be in the matter of the air; and in like manner when food is converted into non-pre-existing man, the form of the man begins to be anew in the matter of the food. Therefore, if bread be changed into the body of Christ, the form of Christ's body must necessarily begin to be in the matter of the bread, which is false. Consequently, the bread is not changed into the substance of Christ's body.
Objection 3. Further, when two things are diverse, one never becomes the other, as whiteness never becomes blackness, as is stated in Phys. i. But since two contrary forms are of themselves diverse, as being the principles of formal difference, so two signate matters are of themselves diverse, as being the principles of material distinction. Consequently, it is not possible for this matter of bread to become this matter whereby Christ's body is individuated, and so it is not possible for this substance of bread to be changed into the substance of Christ's body.
On the contrary, Eusebius Emesenus says: "To thee it ought neither to be a novelty nor an impossibility that earthly and mortal things be changed into the substance of Christ."
I answer that, As stated above (Article 2), since Christ's true body is in this sacrament, and since it does not begin to be there by local motion, nor is it contained therein as in a place, as is evident from what was stated above (1, ad 2), it must be said then that it begins to be there by conversion of the substance of bread into itself.
Yet this change is not like natural changes, but is entirely supernatural, and effected by God's power alone. Hence Ambrose says [(De Sacram. iv): "See how Christ's word changes nature's laws, as He wills: a man is not wont to be born save of man and woman: see therefore that against the established law and order a man is born of a Virgin": and] [The passage in the brackets is not in the Leonine edition] (De Myster. iv): "It is clear that a Virgin begot beyond the order of nature: and what we make is the body from the Virgin. Why, then, do you look for nature's order in Christ's body, since the Lord Jesus was Himself brought forth of a Virgin beyond nature?" Chrysostom likewise (Hom. xlvii), commenting on John 6:64: "The words which I have spoken to you," namely, of this sacrament, "are spirit and life," says: i.e. "spiritual, having nothing carnal, nor natural consequence; but they are rent from all such necessity which exists upon earth, and from the laws here established."
For it is evident that every agent acts according as it is in act. But every created agent is limited in its act, as being of a determinate genus and species: and consequently the action of every created agent bears upon some determinate act. Now the determination of every thing in actual existence comes from its form. Consequently, no natural or created agent can act except by changing the form in something; and on this account every change made according to nature's laws is a formal change. But God is infinite act, as stated in I, 7, 1; 26, 2; hence His action extends to the whole nature of being. Therefore He can work not only formal conversion, so that diverse forms succeed each other in the same subject; but also the change of all being, so that, to wit, the whole substance of one thing be changed into the whole substance of another. And this is done by Divine power in this sacrament; for the whole substance of the bread is changed into the whole substance of Christ's body, and the whole substance of the wine into the whole substance of Christ's blood. Hence this is not a formal, but a substantial conversion; nor is it a kind of natural movement: but, with a name of its own, it can be called "transubstantiation."
Reply to Objection 1. This objection holds good in respect of formal change, because it belongs to a form to be in matter or in a subject; but it does not hold good in respect of the change of the entire substance. Hence, since this substantial change implies a certain order of substances, one of which is changed into the other, it is in both substances as in a subject, just as order and number.
Reply to Objection 2. This argument also is true of formal conversion or change, because, as stated above (ad 1), a form must be in some matter or subject. But this is not so in a change of the entire substance; for in this case no subject is possible.
Reply to Objection 3. Form cannot be changed into form, nor matter into matter by the power of any finite agent. Such a change, however, can be made by the power of an infinite agent, which has control over all being, because the nature of being is common to both forms and to both matters; and whatever there is of being in the one, the author of being can change into whatever there is of being in the other, withdrawing that whereby it was distinguished from the other.
Source: Summa Theologica (New Advent)