Transubstantiation is a concept that Roman Catholic scholastics, most notably Thomas Aquinas, developed for the doctrine of Communion. Catholics state that when a priest blesses the elements of bread and wine, they become the body and blood of Christ.

Catholics admit that chemically they would still be considered bread and wine. But they believe nonetheless that the elements have ceased to be bread and wine and have been converted1 into flesh and blood. They say that the substance has been converted1 (hence the term transubstantiation) but that the accidents of the bread and wine remain as they were. Those accidents include all the ways the senses (touch, taste, etc.) interact with the elements, and all the ways a chemist (for example) would classify the elements in the absence of knowing that a priest has blessed them.

The categories of substance and accidents were borrowed from Aristotle. My question is, is transubstantiation faithful to his categories? I'm not asking if it was intended to be - I think Catholics are aware that they may have borrowed philosophical concepts from others and changed them for their own use - I'm asking if Aristotle's formulation of those concepts is consistent with the philosophical defense of transubstantiation. Specifically, I want to know, if nothing changed outwardly about those elements, could the substance still be said to have changed (from Aristotle's perspective) as Catholics say it could?

1 Originally this question had "transformed" and not "converted." This was sloppy terminology, as transformation would mean a change of shape or form, which is expressly not what happens in transubstantiation. "Conversion" is a better term.

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    In principle, it must be possible to "de-couple" accidents from essence, simply because the accidents are not part of the essence. The issue is that is not at all easy to define the essence of "bread". The predication model of the Categories is not easy to apply to "objects"; consider a chair: what is its essence ? Not the wood or metal it is made of, nor the "shape"... It must be its function. The same for the bread: the ingredients are more or less the same for bread, pizza, cakes... 1/2 Feb 10, 2015 at 9:38
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    Thus we have to conclude that they are all "accidental variations" of the same ur-bread, sharing the same "nutritional" function ? Or are they different substances with different essences ? I think that Aquinas pseudo-Aristotelian theory of transubstantiation was developed exactly "taking benefit" of this "grey area" in A's theory of substance : in most cases, it is not easy to define what the essence is. 2/2 Feb 10, 2015 at 9:42
  • This seems related to my question here: "We know substances by means of their accidents?"
    – Geremia
    Feb 10, 2015 at 17:00
  • @MauroALLEGRANZA That makes a lot of sense (that it's a grey area in Aristotle that Aquinas clarified) and I think your thoughts would make a good answer. Do you think an answer could tackle "To Aquinas, what is the substance/essence of bread?" or would that be better as a separate question? Feb 10, 2015 at 17:01
  • @Geremia Certainly related. Thanks for the link. Feb 10, 2015 at 17:03

1 Answer 1


St. Thomas Aquinas's Summa Theologica question "Of the Accidents Which Remain in This Sacrament" should help.

Relevant to your question "Is transubstantiation faithful to Aristotle's categories?," its article "Whether the accidents remain in this sacrament without a subject?" contains an objection that cites Aristotle:

[N]ot even by miracle can the definition of a thing be severed from it, or the definition of another thing be applied to it; for instance, that, while man remains a man, he can be an irrational animal. For it would follow that contradictories can exist at the one time: for the "definition of a thing is what its name expresses," as is said in Metaph. iv. But it belongs to the definition of an accident for it to be in a subject, while the definition of substance is that it must subsist of itself, and not in another. Therefore it cannot come to pass, even by miracle, that the accidents exist without a subject in this sacrament.

Thus, transubstantiation would seem to contradict Aristotle.

But St. Thomas replies to this objection:

Since being is not a genus, then being cannot be of itself the essence of either substance or accident. Consequently, the definition of substance is not---"a being of itself without a subject," nor is the definition of accident---"a being in a subject"; but it belongs to the quiddity or essence of substance "to have existence not in a subject"; while it belongs to the quiddity or essence of accident "to have existence in a subject." But in this sacrament it is not in virtue of their essence that accidents are not in a subject, but through the Divine power sustaining them; and consequently they do not cease to be accidents, because neither is the definition of accident withdrawn from them, nor does the definition of substance apply to them.

His argument hinges on the real distinction between being and essence, which he overviews in, e.g., De Ente et Essentia.

  • hi sorry to be slow - but you're saying that for aquinas here the essence of an accident is to exist not "be" in the subject; and so through "divine power" accidents need not be there while all the same they continue to exist in the subject: that the chemical make-up of the sacrament is not predicated of anything but still exists in something? it seems interesting, tho i wish you did provide a contemporary / commentarial perspective: is aristotle committed to the definition of 'accident' as being in or existing in the subject of predication?
    – user6917
    Jun 18, 2015 at 10:05
  • is it right to imagine that the disagreement is on whether an accident can exist within a subject while not being predicable of that same subject? it seems so: if an accident need not be in a subject: it need not "be" in any subject of a predication; and so need not "be" in the particular predicative subject which it exists inside; and so need not "be" predicable of that subject [assuming that no thing is predicable that won't "be" in that predication] ... and so (right?) aquinas is saying this mundane chemical make-up exists in the sacrament but needn't be predicable of it?
    – user6917
    Jun 18, 2015 at 10:37

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