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
sacrilege to assert that the Father and the Son are separate in Godhead." We must avoid the adjective "only" [unici] lest we take away the number of
persons. Hence
Hilary says in the same book: "We exclude from
God the
idea of singularity or uniqueness." Nevertheless, we say "the only Son," for in
God there is no plurality of Sons. Yet, we do not say "the only
God," for the Deity is common to several. We avoid the word "confused," lest we take away from the Persons the order of their
nature. Hence
Ambrose says (De Fide i): "What is one is not confused; and there is no multiplicity where there is no difference." The word "solitary" is also to be avoided, lest we take away the society of the three
persons; for, as
Hilary says (De Trin. iv), "We confess neither a solitary nor a diverse
God."
This word "other" [alius], however, in the masculine sense, means only a distinction of "suppositum"; and hence we can properly say that "the Son is other than the Father," because He is another "suppositum" of the divine nature, as He is another person and another hypostasis. 
