_ (other).
But the Son is not alien from the Father, for Hilary says (De Trin.
vii) that "in the divine persons there is nothing diverse, nothing
alien, nothing separable." Therefore the Son is not other than the
Father.

Obj. 4: Further, the terms "other person" and "other thing" [alius et
aliud] have the same meaning, differing only in gender. So if the Son
is another person from the Father, it follows that the Son is a thing
apart from the Father.

_On the contrary,_ Augustine [*Fulgentius, De Fide ad Petrum i.] says:
"There is one essence of the Father and Son and Holy Ghost, in which
the Father is not one thing, the Son another, and the Holy Ghost
another; although the Father is one person, the Son another, and the
Holy Ghost another."

_I answer that,_ Since as Jerome remarks [*In substance, Ep. lvii.], a
heresy arises from words wrongly used, when we speak of the Trinity we
must proceed with care and with befitting modesty; because, as
Augustine says (De Trin. i, 3), "nowhere is error more harmful, the
quest more toilsome, the finding more fruitful." Now, in treating of
the Trinity, we must beware of two opposite errors, and proceed
cautiously between them--namely, the error of Arius, who placed a
Trinity of substance with the Trinity of persons; and the error of
Sabellius, who placed unity of person with the unity of essence.

Thus, to avoid the error of Arius we must shun the use of the terms
diversity and difference in God, lest we take away the unity of
essence: we may, however, use the term "distinction" on account of the
relative opposition. Hence whenever we find terms of "diversity" or
"difference" of Persons used in an authentic work, these terms of
"diversity" or "difference" are taken to mean "distinction." But lest
the simplicity and singleness of the divine essence be taken away, the
terms "separation" and "division," which belong to the parts of a
whole, are to be avoided: and lest quality be taken away, we avoid the
use of the term "disparity": and lest we remove similitude, we avoid
the terms "alien" and "discrepant." For Ambrose says (De Fide i) that
"in the Father and the Son there is no discrepancy, but one Godhead":
and according to Hilary, as quoted above, "in God there is nothing
alien, nothing separable."

To avoid the heresy of Sabellius, we must shun the term "singularity,"
lest we take away the communicability of the divine essence. Hence
Hilary says (De Trin. vii): "It is sacrile