This is your resident Thomist bringing you a minute from the Bellowing Ox.
Many of our Protestant brothers and sisters have accepted, following Luther, the notion of consubstantiation, which means they understand the bread and wine to remain along with Christ’s body and blood at the moment of consecration.
Thomas raises a number of arguments that demonstrate the logical difficulties with such an understanding. First, consubstantiation must assume the Christ comes to the bread and wine as one moves locally from place to place. “Local motion” for Thomas means a body moving through space. If it is true that the bread and wine remain, it follows that Christ must come to the sacrament by local movement. Thomas says this must be the case because a thing only comes to a place that it was not in previously by either moving there locally (i.e. from point A to point B), or by conversion of another thing into itself.
Christ cannot come to the Eucharist locally because it would follow that He would then not be in heaven and in the Eucharist. It would also follow that He could not be in many places at once. If He’s moving from place to place, He can only be in one place at a time. Hence, the whole substance must change, and wholly so.
Secondly, Thomas says consubstantiation is not scriptural as the words in the Bible should read “Here is my body,” instead of “This is my body.” Thirdly, he says we could not “adore” the Eucharist if the bread remained, as it would be an idol. Finally, he cites church law that forbids eating food before taking the Eucharist. This forbidding would be in vain if bread remained because natural food would be present with the supernatural food of Christ’s body.