Image: St Thomas Aquinas presenting his work to Our Lady. Aristotle is in the foreground. The Latin text reads, “You have written well of me, Thomas.” Photograph is mine, taken in the Vatican Palaces.
This is the second installment in a three-part series on the doctrine of Transubstantiation in the sacramental theology of Thomas Aquinas.
In the following articles, Thomas deals with the change in the substance of the bread and wine into the substance of Christ’s body and blood. In answer to the question of whether the substance of bread and wine remain in the sacrament, Thomas appeals once again to the fact that no local presence is involved, and he further details what this means in three points:
Now it is evident that Christ’s body does not begin to be present in this sacrament by local motion. First of all, because it would follow that it would cease to be in heaven: for what is moved locally does not come anew to some place unless it quit the former one. Secondly, because every body moved locally passes through all intermediary spaces, which cannot be said here. Thirdly, because it is not possible for one movement of the same body moved locally to be terminated in different places at the one time, whereas the body of Christ under this sacrament begins at the one time to be in several places.
Thus, as we saw before with his use of Augustine, he affirms that the ascended body of Christ remains in heaven and does not move through space and time to be present on the altars across the world. Yet, if there is no change of place by which the substance of the bread and wine are removed and replaced by the body-blood of Christ, then the only other alternative is that there is a change of substance which does not require such displacement of location. This change of substance is the doctrine of Transubstantiation. In the fifth article, Thomas gives one of his clearest expositions of what it means to have the substance of a thing changed, while the accidents remain. The substance is that which makes bread to be bread; that which makes wine to be wine. The accidents (or species) are that by which the sense perception determines a thing to be bread or a human body, et cetera. Any given thing reveals itself to our understanding through its physical properties which we engage through sense experience. The “what-ness” (substance) of a thing is not reducible to its accidents, yet the union of substance and accidents is inviolable in the Greek understanding. Aristotle never supposed that there are instances of a substance utilizing the accidents of a different substance. Yet, this is precisely what the doctrine of Transubstantiation claims. Hence, Thomas emphasizes, in the fourth article, that this is an act of divine power, not the coercion of immanent natural properties.
Thus, in the fifth article, in answer to the question of whether the accidents of the bread and wine remain after the consecration, Thomas affirms that all of the accidents remain. This means that at every point in the human perception of the bread’s material existence, it is bread. In a chemistry lab today, it would mean that the chemical composition of the bread under the microscope would be exactly the same prior to the consecration as afterwards. If the accidents were to change along with the substance according to the species proper to the substance, then the body and blood of Christ would appear as it did to his disciples and as it exists now in heaven. Yet, in so many words, Thomas rebukes this as cannibalism. He remarks:
It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh and to drink blood. And therefore Christ’s flesh and blood are set before us to be partaken of under the species of those things which are the more commonly used by men, namely, bread and wine. Secondly, lest this sacrament might be derided by unbelievers, if we were to eat our Lord under His own species. Thirdly, that while we receive our Lord’s body and blood invisibly, this may redound to the merit of faith.
Thus, Thomas quite clearly rejects any claim that the Eucharist involves the eating of human flesh and the drinking of human blood, as would be required by the normal union of accidents that are proper to a substance – which would be “horrible” and cause “derision.” Yet, there is still truly a reception of Christ’s body and blood, but it is “invisible.” This invisibility is not a trick of our perception; rather, it is the consequence of the fact that substance itself is not visible. The accidents are what are visible. The material existence of the post-consecrated “bread” is not merely a matter of perception, as this would have more in common with modern Idealist philosophy than with Thomas’ realism. Ontology, both physical and metaphysical, precedes epistemology in Thomas’ realist mindset. Thus, the accidents of the bread and wine are really there; that is, the material and local presence of the bread and wine are truly there, just as before. Yet, bread and wine can no longer be predicated of these elements, since what they “are” is the body and blood of Christ. Once again, the substance of a thing is what it truly is. Under normal circumstances, this substance requires physical extension that is proper to it, but in the Eucharist the substance of Christ’s body and blood utilizes the extended (material) properties that are normally proper to bread and wine.
In the subsequent section (question 76), Thomas once again objects to the idea that Christ’s body is in this sacrament as in a place (article five). He uses the distinctions already discussed to argue that Christ’s body “is in heaven under its own species, and on many altars under sacramental species…not in this sacrament circumscriptively, because it is not there according to the commensuration of its own quantity, as stated above.” Moreover, in the following article, Thomas rejects the idea that Christ’s body is in this sacrament movably: “But Christ’s body is at rest in heaven. Therefore it is not movably in this sacrament.” He then appeals to the previous article, noting that such mobility requires local presence. Repeatedly Thomas has affirmed that Christ’s body is in heaven, and the bodily presence in heaven is “under its own species,” which is to say that it retains the accidental properties that are proper to it: those of the resurrected and glorified body of Christ. This bodily presence in heaven is distinct from the bodily presence on the altars. In the former, the accidents are those proper to the substance; in the latter, the accidents are those of bread and wine, the substances of which no longer exist. As noted above, there is no analogy to this in the natural sphere. Thus, transubstantiation – an instance where the substance changes while the accidents remain – is an entirely new phenomenon. While the categories of substance and accidents may be dependent on Greek philosophy, their usage is in service to a work of God without parallel in the rest of creation. In this way, Thomas is utilizing concepts which, in their original setting, would deny the possibility of what he is describing.
 ST IIIa, Q75, A2.
 ST IIIa, Q75, A5.
 ST IIIa, Q76, A5, ad. 1.
 ST IIIa, Q76, A6.