This is your resident Thomist bringing you a minute from the Bellowing Ox.
Did you know that the terminology “transubstantiation” is actually a metaphysical concept borrowed from Aristotelian categories? Aquinas coined the phrase to capture what exactly happens at the moment the priest says, “This is my body.”
So what is substance? From its Latin etymology it literally means “That which stands under.” Aquinas himself calls it the “intelligible content” of a thing. When he says it is “intelligible” he means that it cannot be seen with sense powers. You cannot see or touch substance, you can only understand it. If substance could be seen, impersonation would be impossible. It would be impossible, for instance, for identical twins to trick their parents by calling each other by the other’s name if substance could be seen.
Given this idea, transubstantiation, or the changing of the bread and wine into the body and blood of Christ doesn’t seem so farfetched does it? That said, Thomas is careful to show that belief in transubstantiation is a matter of faith and not of understanding. Since we can’t “see” substance, only “understand it” it makes it easier to have faith in the Eucharist, does it not?