Objection 1. It seems that this proposition is false: "The body of Christ is made out of bread." For everything out of which another is made, is that which is made the other; but not conversely: for we say that a black thing is made out of a white thing, and that a white thing is made black: and although we may say that a man becomes black still we do not say that a black thing is made out of a man, as is shown in Phys. i. If it be true, then, that Christ's body is made out of bread, it will be true to say that bread is made the body of Christ. But this seems to be false, because the bread is not the subject of the making, but rather its term. Therefore, it is not said truly that Christ's body is made out of bread.
Objection 2. Further, the term of "becoming" is something that is, or something that is "made." But this proposition is never true: "The bread is the body of Christ"; or "The bread is made the body of Christ"; or again, "The bread will be the body of Christ." Therefore it seems that not even this is true: "The body of Christ is made out of bread."
Objection 3. Further, everything out of which another is made is converted into that which is made from it. But this proposition seems to be false: "The bread is converted into the body of Christ," because such conversion seems to be more miraculous than the creation of the world, in which it is not said that non-being is converted into being. Therefore it seems that this proposition likewise is false: "The body of Christ is made out of bread."
Objection 4. Further, that out of which something is made, can be that thing. But this proposition is false: "Bread can be the body of Christ." Therefore this is likewise false: "The body of Christ is made out of bread."
On the contrary, Ambrose says (De Sacram. iv): "When the consecration takes place, the body of Christ is made out of the bread."
I answer that, This conversion of bread into the body of Christ has something in common with creation, and with natural transmutation, and in some respect differs from both. For the order of the terms is common to these three; that is, that after one thing there is another (for, in creation there is being after non-being; in this sacrament, Christ's body after the substance of bread; in natural transmutation white after black, or fire after air); and that the aforesaid terms are not coexistent.
Now the conversion, of which we are speaking, has this in common with creation, that in neither of them is there any common subject belonging to either of the extremes; the contrary of which appears in every natural transmutation.
Again, this conversion has something in common with natural transmutation in two respects, although not in the same fashion. First of all because in both, one of the extremes passes into the other, as bread into Christ's body, and air into fire; whereas non-being is not converted into being. But this comes to pass differently on the one side and on the other; for in this sacrament the whole substance of the bread passes into the whole body of Christ; whereas in natural transmutation the matter of the one receives the form of the other, the previous form being laid aside. Secondly, they have this in common, that on both sides something remains the same; whereas this does not happen in creation: yet differently; for the same matter or subject remains in natural transmutation; whereas in this sacrament the same accidents remain.
From these observations we can gather the various ways of speaking in such matters. For, because in no one of the aforesaid three things are the extremes coexistent, therefore in none of them can one extreme be predicated of the other by the substantive verb of the present tense: for we do not say, "Non-being is being" or, "Bread is the body of Christ," or, "Air is fire," or, "White is black." Yet because of the relationship of the extremes in all of them we can use the preposition "ex" [out of, which denotes order; for we can truly and properly say that "being is made out of non-being," and "out of bread, the body of Christ," and "out of air, fire," and "out of white, black." But because in creation one of the extremes does not pass into the other, we cannot use the word "conversion" in creation, so as to say that "non-being is converted into being": we can, however, use the word in this sacrament, just as in natural transmutation. But since in this sacrament the whole substance is converted into the whole substance, on that account this conversion is properly termed transubstantiation.
Again, since there is no subject of this conversion, the things which are true in natural conversion by reason of the subject, are not to be granted in this conversion. And in the first place indeed it is evident that potentiality to the opposite follows a subject, by reason whereof we say that "a white thing can be black," or that "air can be fire"; although the latter is not so proper as the former: for the subject of whiteness, in which there is potentiality to blackness, is the whole substance of the white thing; since whiteness is not a part thereof; whereas the subject of the form of air is part thereof: hence when it is said, "Air can be fire," it is verified by synecdoche by reason of the part. But in this conversion, and similarly in creation, because there is no subject, it is not said that one extreme can be the other, as that "non-being can be being," or that "bread can be the body of Christ": and for the same reason it cannot be properly said that "being is made of [de] non-being," or that "the body of Christ is made of bread," because this preposition "of" [de] denotes a consubstantial cause, which consubstantiality of the extremes in natural transmutations is considered according to something common in the subject. And for the same reason it is not granted that "bread will be the body of Christ," or that it "may become the body of Christ," just as it is not granted in creation that "non-being will be being," or that "non-being may become being," because this manner of speaking is verified in natural transmutations by reason of the subject: for instance, when we say that "a white thing becomes black," or "a white thing will be black."
Nevertheless, since in this sacrament, after the change, something remains the same, namely, the accidents of the bread, as stated above (Article 5), some of these expressions may be admitted by way of similitude, namely, that "bread is the body of Christ," or, "bread will be the body of Christ," or "the body of Christ is made of bread"; provided that by the word "bread" is not understood the substance of bread, but in general "that which is contained under the species of bread," under which species there is first contained the substance of bread, and afterwards the body of Christ.
Reply to Objection 1. That out of which something else is made, sometimes implies together with the subject, one of the extremes of the transmutation, as when it is said "a black thing is made out of a white one"; but sometimes it implies only the opposite or the extreme, as when it is said--"out of morning comes the day." And so it is not granted that the latter becomes the former, that is, "that morning becomes the day." So likewise in the matter in hand, although it may be said properly that "the body of Christ is made out of bread," yet it is not said properly that "bread becomes the body of Christ," except by similitude, as was said above.
Reply to Objection 2. That out of which another is made, will sometimes be that other because of the subject which is implied. And therefore, since there is no subject of this change, the comparison does not hold.
Reply to Objection 3. In this change there are many more difficulties than in creation, in which there is but this one difficulty, that something is made out of nothing; yet this belongs to the proper mode of production of the first cause, which presupposes nothing else. But in this conversion not only is it difficult for this whole to be changed into that whole, so that nothing of the former may remain (which does not belong to the common mode of production of a cause), but furthermore it has this difficulty that the accidents remain while the substance is destroyed, and many other difficulties of which we shall treat hereafter (77). Nevertheless the word "conversion" is admitted in this sacrament, but not in creation, as stated above.
Reply to Objection 4. As was observed above, potentiality belongs to the subject, whereas there is no subject in this conversion. And therefore it is not granted that bread can be the body of Christ: for this conversion does not come about by the passive potentiality of the creature, but solely by the active power of the Creator.
Source: Summa Theologica (New Advent)