e is called by the
Greeks _ousia,_ what we may call "essence." In another sense
substance means a subject or _suppositum,_ which subsists in the
genus of substance. To this, taken in a general sense, can be
applied a name expressive of an intention; and thus it is called
_suppositum._ It is also called by three names signifying a
reality--that is, "a thing of nature," "subsistence," and
"hypostasis," according to a threefold consideration of the substance
thus named. For, as it exists in itself and not in another, it is
called "subsistence"; as we say that those things subsist which exist
in themselves, and not in another. As it underlies some common
nature, it is called "a thing of nature"; as, for instance, this
particular man is a human natural thing. As it underlies the
accidents, it is called "hypostasis," or "substance." What these
three names signify in common to the whole genus of substances, this
name "person" signifies in the genus of rational substances.

Reply Obj. 1: Among the Greeks the term "hypostasis," taken in the
strict interpretation of the word, signifies any individual of the
genus substance; but in the usual way of speaking, it means the
individual of the rational nature, by reason of the excellence of
that nature.

Reply Obj. 2: As we say "three persons" plurally in God, and "three
subsistences," so the Greeks say "three hypostases." But because the
word "substance," which, properly speaking, corresponds in meaning to
"hypostasis," is used among us in an equivocal sense, since it
sometimes means essence, and sometimes means hypostasis, in order to
avoid any occasion of error, it was thought preferable to use
"subsistence" for hypostasis, rather than "substance."

Reply Obj. 3: Strictly speaking, the essence is what is expressed by
the definition. Now, the definition comprises the principles of the
species, but not the individual principles. Hence in things composed
of matter and form, the essence signifies not only the form, nor only
the matter, but what is composed of matter and the common form, as
the principles of the species. But what is composed of this matter
and this form has the nature of hypostasis and person. For soul,
flesh, and bone belong to the nature of man; whereas this soul, this
flesh and this bone belong to the nature of this man. Therefore
hypostasis and person add the individual principles to the idea of
essence; nor are these identified with the essence in things compose