While I'm on the subject of Catholicism, I thought I'd post my opening statement in a debate I had over transubstantiation. I opted to use only philosophical arguments in this debate. The resolution for this debate was The Catholic doctrine of transubstantiation is false. Here's my opening.
According to the doctrine of transubstantiation, when the bread and wine of the eucharist are consecrated, they cease to be bread and wine and literally become the body and blood of Jesus (Catechism of the Catholic Church, section 1376).
To keep things short, I'll refer just to "the bread" instead of "the bread and wine," but whatever I say about the bread should apply just as well to the wine.
There's only one of two ways that transubstantiation could happen. Either the material from which the bread is made (i.e., the molecules, atoms, and subatomic particles) remain there but rearrange themselves so that they cease to be bread and become human flesh, or all of the material from which the bread is made is completely replaced by the material of Jesus' flesh, kind of how wood is petrified by replacing molecule after molecule of wood with molecules after molecule of mineral until there is no longer any wood, but just mineral where the tree used to be.
Either scenario creates problems with identity. By "problem with identity," I mean the problem of what makes the flesh in the Eucharist actually be Jesus' own flesh rather than a replication of Jesus' flesh or somebody else's flesh.
Rearrangement of parts
Let's say transubstantiation works by the bread turning into human flesh by the parts rearranging themselves. Obviously, the bread was not Jesus' flesh before the transubstantiation. So there's nothing that could be done to the bread to make it Jesus' flesh. If none of the parts are members of Jesus' flesh before the transformation, then none of the parts could be members of Jesus' flesh after the transformation. No rearrangement of the parts could cause it to be Jesus' flesh.
If a scientist was able to grow skin in a lab that happened to resemble your flesh down to the DNA structure, but he didn't use any of your actual body parts to grow it from, but rather made it from scratch, then it could not be your flesh. The only way it could be your flesh is if you wore it and it was attached to the rest of the actual body you are animating. If it is completely discontinuous with your body, then it can't be your body. It would be like a person painting a perfect replica of the Mona Lisa. The replica would not be the Mona Lisa itself, but only a perfect duplicate.
Flesh ex nihilo
Now, let's say transubstantiation works by completely replacing all the parts of the bread with human flesh. This creates pretty much the same problem. In this case, the bread basically ceases to exist, and human flesh is created ex-nihilo where the bread used to be. This happens so quickly that nobody who was watching could tell. If that were to happen, then the flesh that came into existence could not be Jesus' flesh. It could, at best, be a perfect replica of Jesus' flesh. Imagine a wizard causes a cat to pop into existence out of thin air, and imagine the cat is an exact duplicate of another cat down to the molecule. It would still not be the same cat because one cat can't be in two different places at the same time. It would just be a duplicate. in the same way, if human flesh pops into existence that's just like the flesh of Jesus himself, then it still can't be Jesus' flesh. At best, it can only be a duplication of Jesus' flesh.
Conservation of mass
The first transubstantiation was supposed by the Catholic Church to have happened at the last supper when Jesus broke the bread, said, "This is my body," and passed it around for the disciples to eat (Luke 22:19). There is no indication in the story that Jesus lost any body parts during this meal, and I've never heard a Catholic claim that's what happened. Jesus' actual body was located at a specific place in space during this meal, so if the bread became human flesh, by either of the means mentioned above, then it could not have been Jesus' flesh. Assuming it became human flesh, that would mean more human flesh came into existence than was there before, and since all of Jesus' flesh was accounted for by being attached to the body he was then animating, the flesh being passed around for consumption could not have been his flesh.
It's even worse today. Mass takes place all over the world on Sunday, and there are probably tons of wafers that are all supposedly transubstantiated. Jesus' physical body is made of a particular amount of stuff--roughly the same amount as any man. So it is impossible that all those millions of wafers around the world could all be Jesus' body. There isn't enough of Jesus' actual body to go around.
Properties
But the problem is worse than that because the bread of the eucharist is not human flesh at all because it doesn't have any human properties. Now, Catholics are right to make a distinction between essential properties and accidental properties. An accidental property would be like a round ball of wax being shaped into a cube. Although the wax changes shape, it remains the same piece of wax. So the shape of the wax is an accidental property. An essential property would be what makes it wax. If you changed the chemical composition, then it would no longer be wax.
Catholics claim that during the process of transubstantiation, the flesh retains the accidental properties of bread, but it has the essential properties of Jesus' flesh. But the problem is that it retains all of the properties of bread and it gains none of the properties of human flesh. To be human flesh, it would have to have a human cellular structure with human DNA. But the bread of the eucharist does not. It is impossible for it to be human flesh since it has none of the properties of human flesh.
Imagine if I told you that your computer was actually a Christmas tree, but that it just happened to have all the properties of a computer and none of the properties of a Christmas tree. That would obviously be absurd. Or, imagine I told you that a square could have all the properties of a circle and none of the properties of a square and still be a square. You'd know immediately that I was talking crazy. In the same way, if the eucharist has all the properties of bread and none of the properties of human flesh, then it can't be human flesh.
Conclusion
Therefore, it's impossible for the doctrine of transubstantiation to be true.
Thank you.