a. About the Possibility of Transubstantiation

144. About the first, one needs to know: either that for the transubstantiation of one thing into another no more is required save that each term is a substance and is fully under obedience to the agent as to total being and total non-being; or that, according to others (namely Godfrey of Fontaines Quodlibet V q.1), more is required, namely mutual convertibility in nature (because, that is, of identity in the principle of potential transmutability from the form of one thing to the form of another, although not by a single transmutation but by many, and this by a natural agent). One needs to know which of these is kept to in the question at issue, because both the bread and the body of Christ are totally in the power of God as to total being and total non-being.

145. They [the bread and Christ’s body] also have matter of the same idea, through which, on the part of each term, a transition can be made by nature from one to the other, and that through many intermediate stages, although considering Christ’s body insofar as it is now immortal and impassible, it could not be converted by nature into bread. But this is not because it lacks matter receptive of the form of bread, but because its matter is contained inseparably by its form, namely the impassible soul.

146. This could also be made clear about the parts of each term. For God has in his power both the matter and the form, and can consequently convert matter into matter and form into form, and thus whole into whole, and totally. But a natural agent cannot do this, because matter is not in its power but is presupposed to its action, from Ord. IV d.1 nn.141-145.

147. This reasoning proceeds according to the way touched on in the immediately preceding question [n.61], but without supposing that conversion requires a receptive subject common to each term.