ia_, [Greek: ousiosis]
for _subsistentia_, [Greek: hupostasis] for _substantia_,
[Greek: prosopon] for _persona_. But the Greeks called individual
substances [Greek: hupostaseis] because they underlie the rest and offer
support and substrate to what are called accidents; and we in our term
call them substances as being substrate--[Greek: hupostaseis], and since
they also term the same substances [Greek: prosopa], we too may call
them persons. So [Greek: ousia] is identical with essence, [Greek:
ousiosis] with subsistence, [Greek: hupostasis] with substance, [Greek:
prosopon] with person. But the reason why the Greek does not use [Greek:
hupostasis] of irrational animals while we apply the term substance to
them is this: This term was applied to things of higher value, in order
that what is more excellent might be distinguished, if not by a
definition of nature answering to the literal meaning of [Greek:
huphistasthai]=_substare_, at any rate by the words [Greek:
hupostasis]=_substantia_.

To begin with, then, man is essence, i.e. [Greek: ousia], subsistence,
i.e. [Greek: ousiosis, hupostasis], i.e. substance, [Greek: prosopon],
i.e. person: [Greek: ousia] or _essentia_ because he is, [Greek:
ousiosis], or subsistence because he is not accidental to any subject,
[Greek: hupostusis] or substance because he is subject to all the things
which are not subsistences or [Greek: ousioseis], while he is [Greek:
prosopon] or person because he is a rational individual. Next, God is
[Greek: ousia], or essence, for He is and is especially that from which
proceeds the Being of all things. To Him belong [Greek: ousiosis], i.e.
subsistence, for He subsists in absolute independence, and [Greek:
huphistasthai], for He is substantial Being. Whence we go on to say that
there is one [Greek: ousia] or [Greek: ousiosis], i.e. one essence or
subsistence of the Godhead, but three [Greek: hupostaseis] or
substances. And indeed, following this use, men have spoken of One
essence, three substances and three persons of the Godhead. For did not
the language of the Church forbid us to say three substances in speaking
of God,[63] substance might seem a right term to apply to Him, not
because He underlies all other things like a substrate, but because,
just as He excels above all things, so He is the foundation and support
of things, supplying them all with [Greek: ousiosthai] or subsi