"principle from a principle."
"Union" implies the unity of two; and is therefore appropriated to the
Holy Ghost, inasmuch as He proceeds from two. And from this we can
understand what Augustine means when he says (De Doctr. Christ. i, 5)
that "The Three are one, by reason of the Father; They are equal by
reason of the Son; and are united by reason of the Holy Ghost." For it
is clear that we trace a thing back to that in which we find it first:
just as in this lower world we attribute life to the vegetative soul,
because therein we find the first trace of life. Now "unity" is
perceived at once in the person of the Father, even if by an
impossible hypothesis, the other persons were removed. So the other
persons derive their unity from the Father. But if the other persons
be removed, we do not find equality in the Father, but we find it as
soon as we suppose the Son. So, all are equal by reason of the Son,
not as if the Son were the principle of equality in the Father, but
that, without the Son equal to the Father, the Father could not be
called equal; because His equality is considered firstly in regard to
the Son: for that the Holy Ghost is equal to the Father, is also from
the Son. Likewise, if the Holy Ghost, Who is the union of the two, be
excluded, we cannot understand the oneness of the union between the
Father and the Son. So all are connected by reason of the Holy Ghost;
because given the Holy Ghost, we find whence the Father and the Son
are said to be united.

According to the third consideration, which brings before us the
adequate power of God in the sphere of causality, there is said to be
a third kind of appropriation, of "power," "wisdom," and "goodness."
This kind of appropriation is made both by reason of similitude as
regards what exists in the divine persons, and by reason of
dissimilitude if we consider what is in creatures. For "power" has
the nature of a principle, and so it has a likeness to the heavenly
Father, Who is the principle of the whole Godhead. But in an earthly
father it is wanting sometimes by reason of old age. "Wisdom" has
likeness to the heavenly Son, as the Word, for a word is nothing but
the concept of wisdom. In an earthly son this is sometimes absent by
reason of lack of years. "Goodness," as the nature and object of love,
has likeness to the Holy Ghost; but seems repugnant to the earthly
spirit, which often implies a certain violent impulse, according to
Isa. 25:4: "The spirit of