rms, because, since these forms,
considered in themselves, are naturally in something as in a subject,
from the very fact that one of them is received in matter, which is
not in another, it follows that neither can the form itself thus
existing be in another. As to the second, it must be maintained that
the principle of individuation is dimensive quantity. For that
something is naturally in another one solely, is due to the fact that
that other is undivided in itself, and distinct from all others. But
it is on account of quantity that substance can be divided, as is
said in _Phys._ i. And therefore dimensive quantity itself is a
particular principle of individuation in forms of this kind, namely,
inasmuch as forms numerically distinct are in different parts of the
matter. Hence also dimensive quantity has of itself a kind of
individuation, so that we can imagine several lines of the same
species, differing in position, which is included in the notion of
this quantity; for it belongs to dimension for it to be "quantity
having position" (Aristotle, Categor. iv), and therefore dimensive
quantity can be the subject of the other accidents, rather than the
other way about.

Reply Obj. 1: One accident cannot of itself be the subject of
another, because it does not exist of itself. But inasmuch as an
accident is received in another thing, one is said to be the subject
of the other, inasmuch as one is received in a subject through
another, as the surface is said to be the subject of color. Hence
when God makes an accident to exist of itself, it can also be of
itself the subject of another.

Reply Obj. 2: The other accidents, even as they were in the substance
of the bread, were individuated by means of dimensive quantity, as
stated above. And therefore dimensive quantity is the subject of the
other accidents remaining in this sacrament, rather than conversely.

Reply Obj. 3: Rarity and density are particular qualities
accompanying bodies, by reason of their having much or little matter
under dimensions; just as all other accidents likewise follow from
the principles of substance. And consequently, as the accidents are
preserved by Divine power when the substance is withdrawn, so, when
matter is withdrawn, the qualities which go with matter, such as
rarity and density, are preserved by Divine power.

Reply Obj. 4: Mathematical quantity abstracts not from intelligible
matter, but from sensible matter, as is said in _Metaph._