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In Aristotle's philosophy, physical objects have a substance and accidents. A substance is what the object "really is" and the accidents are things that can change without changing the object itself. As a loose analogy, my car's substance is a transportation vehicle, and its accidents are things like color, power, materials, etc. My blue car can change into a green car and it'll still be a car. The accidental property of color changed, but the car remained substantially the same.

Catholic theology uses this distinction to explain a doctrine known as "transubstantiation". Transubstantiation is the belief that after the priest performs a ritual over a piece of bread, the bread becomes a new object that is both a man and God. However, all the physical properties remain the same. Where the object to be examined scientifically, the chemical properties would be indistinguishable from bread. The reason why we Catholics believe we can still say the object is actually a God man rather than a piece of bread is due to the Aristotelean distinction between substance and accidents. The substance of the object has changed from bread into God man, while the accidentals that make the object physically indistinguishable from a piece of bread remain the same.

Modern physics says a physical object is just a collection of physical units, whether we call those smallest physical units atoms, quarks, or quantum wavefunctions. It's like saying everything in the world is made out of legos, and a lego piece of bread is just the collection of bricks from which it is made. If all the bricks are removed from the lego piece of bread, then no piece of bread remains. Therefore, the bread is nothing more than the bricks. This also means there is nothing about the bread that can change besides the bricks. If something does change about the bread, then something must change in the bricks. In the lego universe, change is completely brickified.

Bringing this back to Aristotelean substances, then within the lego universe, substances do not exist. There is no true distinction between what a lego object really is and the bricks it is made of. If all the accidents of a lego object are removed by removing all the bricks, nothing of the lego object remains in the lego universe. Likewise with the view of modern physics, when we remove all the smallest physical units from some physical object, nothing of the object remains. Whatever the object is is entirely composed by the collection of smallest physical units. Consequently, within the framework of modern physics, it appears the Aristotelean distinction between substance and accidents does not describe reality, and is contradicted. At best, we might use the distinction as a heuristic to make it easier to mentally describe physical objects, but the distinction remains a useful labelling system, and does not describe objects as they really are.

Applying this to the Catholic doctrine of transubstantiation, if the above is true, then this also implies there is no substance in a piece of bread that can be changed from bread into God man without affecting the accidental physical qualities of the bread. If any change does occur, then it necessitates the physical constituents of the bread also change. Conversely, if no change occurs to the physical constituents, then nothing about the bread changes, and it remains a piece of bread.

So, my question is twofold:

  1. Is my understanding of the Aristotelean distinction between substance and accidents correct?
  2. Is it true that modern physics contradicts this distinction? If not, how is the contradiction avoided?
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  • I don't have an answer at the moment for #1 or #2, but I suggest 3: when one tries to use a thing to meditate between abstractions, it usually does not go well.
    – Scott Rowe
    Sep 25, 2022 at 21:25
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    I think you are confusing substance with essence, see SEP, Essential vs. Accidental Properties. On Aristotle's conception, substance is a compound of both with "matter". On how it compares to prevailing views of modern physics see Is physicalism ultimately essentialist? However, modern physics, unlike physicalism or some other metaphysical interpretation, only concerns empirical matters and can be reconciled with transubstantiation if need be.
    – Conifold
    Sep 25, 2022 at 23:45
  • At least in school I was taught that physical things are made up of particles and fields. Then later on I was taught these particles were produced by quantum wavefunctions collapsing. However, all of these renderings of physics reduce everything physical to some kind of lego. We don't seem to have detected anything else of which physical objects are composed. I don't understand when you say physics "only concerns empirical matters" as if that somehow resolves the problem. I could say each of us is part of the Sky Banana, which is empirically undetectable, but that doesn't make it so.
    – yters
    Sep 26, 2022 at 1:44

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