a kind of change. But in every change there
must be some subject, which from being previously in potentiality is
now in act. because as is said in _Phys._ iii: "motion is the act of
a thing existing in potentiality." But no subject can be assigned for
the substance of the bread and of the body of Christ, because it is
of the very nature of substance for it "not to be in a subject," as
it is said in _Praedic._ iii. Therefore it is not possible for the
whole substance of the bread to be converted into the body of Christ.

Obj. 2: Further, the form of the thing into which another is
converted, begins anew to inhere in the matter of the thing converted
into it: as when air is changed into fire not already existing, the
form of fire begins anew to be in the matter of the air; and in like
manner when food is converted into non-pre-existing man, the form of
the man begins to be anew in the matter of the food. Therefore, if
bread be changed into the body of Christ, the form of Christ's body
must necessarily begin to be in the matter of the bread, which is
false. Consequently, the bread is not changed into the substance of
Christ's body.

Obj. 3: Further, when two things are diverse, one never becomes the
other, as whiteness never becomes blackness, as is stated in _Phys._
i. But since two contrary forms are of themselves diverse, as being
the principles of formal difference, so two signate matters are of
themselves diverse, as being the principles of material distinction.
Consequently, it is not possible for this matter of bread to become
this matter whereby Christ's body is individuated, and so it is not
possible for this substance of bread to be changed into the substance
of Christ's body.

_On the contrary,_ Eusebius Emesenus says: "To thee it ought neither
to be a novelty nor an impossibility that earthly and mortal things
be changed into the substance of Christ."

_I answer that,_ As stated above (A. 2), since Christ's true body is
in this sacrament, and since it does not begin to be there by local
motion, nor is it contained therein as in a place, as is evident from
what was stated above (A. 1, ad 2), it must be said then that it
begins to be there by conversion of the substance of bread into
itself.

Yet this change is not like natural changes, but is entirely
supernatural, and effected by God's power alone. Hence Ambrose says
[(De Sacram. iv): "See how Christ's word changes nature's laws, as He
wills: a man is