This is my body, this is my blood (the quote is below, relax). Thomas Aquinas took that literally, and then ran into problems with his systematic theology which is based on Aristotle and the idea of forms. Which leads to this contradiction.
St. Thomas finds himself in a pickle. Supporting wholeheartedly the clear and constant profession of the Catholic Church, he affirms repeatedly that the bread and wine are indispensable to the efficacy of the sacrament, and he also must affirm, by the Aristotelian form-matter analysis that he wants to deploy, that as material causes, the bread and wine are completely dispensable.
According to the metaphysics of Aristotle, the bread and wine can be any material whatever, but according to the Catholic Church, this is a complete falsity, a gigantic heresy, an outright absurdity, with numerous and deep implications for the whole of Catholic profession and life. It’s difficult to imagine saying that “in principle” the bread and wine could be anything whatever, and remaining any kind of Catholic at all. It’s that far removed from the “this-ness” of real Catholicism.
But this is precisely what the Aristotelian metaphysics forces. So (following his Catholic faith) St. Thomas says that the bread and wine are very specifically indispensable to the sacrament, but (following Aristotle) he must simultaneously say that it is impossible for the bread and wine to be indispensable; for they lack a formal “subject of inherence.”
This contradiction, out of St. Thomas’s own mouth, and regarding the Eucharist itself, the very heart of the Catholic Church, refutes “Thomism” root and branch. The Aristotelian form-matter analysis is radically insufficient to Catholic theology. The proof by contradiction is complete.
It is true that I have never read, nor met, any Thomist dumb enough to fall for such a simple, clear, “inclined plane”, “balls dropped from the tower” refutation of an entire theory. Thomists are far too intelligent, too highly trained, even to be bothered, let alone intrigued, by a parlor trick.
Except it is not. Christ said of the bread "this is my body broken for you" as he sat, incarnate in a body, at the last supper. He did the same as the wine. These things are metaphors.
Calvin is more correct here.
Now, as the knowledge of this great mystery is most necessary, and, in proportion to its importance, demands an accurate exposition, and Satan, in order to deprive the Church of this inestimable treasure, long ago introduced, first, mists, and then darkness, to obscure its light, and stirred up strife and contention to alienate the minds of the simple from a relish for this sacred food, and in our age, also, has tried the same artifice, I will proceed, after giving a simple summary adapted to the capacity of the ignorant, to explain those difficulties by which Satan has tried to ensnare the world. First, then, the signs are bread and wine, which represent the invisible food which we receive from the body and blood of Christ. For as God, regenerating us in baptism, ingrafts us into the fellowship of his Church, and makes us his by adoption, so we have said that he performs the office of a provident parent, in continually supplying the food by which he may sustain and preserve us in the life to which he has begotten us by his word. Moreover, Christ is the only food of our soul, and, therefore, our heavenly Father invites us to him, that, refreshed by communion with him, we may ever and anon gather new vigour until we reach the heavenly immortality. But as this mystery of the secret union of Christ with believers is incomprehensible by nature, he exhibits its figure and image in visible signs adapted to our capacity, nay, by giving, as it were, earnests and badges, he makes it as certain to us as if it were seen by the eye; the familiarity of the similitude giving it access to minds however dull, and showing that souls are fed by Christ just as the corporeal life is sustained by bread and wine. We now, therefore, understand the end which this mystical benediction has in view—viz. to assure us that the body of Christ was once sacrificed for us, so that we may now eat it, and, eating, feel within ourselves the efficacy of that one sacrifice,—that his blood was once shed for us so as to be our perpetual drink. This is the force of the promise which is added, “Take, eat; this is my body, which is broken for you” (Mt. 26:26, &c.). The body which was once offered for our salvation we are enjoined to take and eat, that, while we see ourselves made partakers of it, we may safely conclude that the virtue of that death will be efficacious in us. Hence he terms the cup the covenant in his blood. For the covenant which he once sanctioned by his blood he in a manner renews, or rather continues, in so far as regards the confirmation of our faith, as often as he stretches forth his sacred blood as drink to us.
What both Aquinas and Calvin are trying to do is stick close to the text of the gospel and the practice of Christ. The comments at Briggs place are useful -- because all admit that the Aristotalean model breaks down.
Of course, I found Calvin more sensible here, though in the remnant of his chapter he pulls out his metaphorical cane and gives the Schoolmen (Thomists) a rhetorical thrashing.
As I've said before, We cannot understand the mind of God. We have what we are given, and what is declared. Our speculations are too often erroneous. Stay close to the word of God, and all well be well.