The Theological Tractates and The Consolation of Philosophy · Boethius
VI.
But since no relation can be affirmed of one subject alone, since a predication referring to one substance is a predication without relation, the manifoldness of the Trinity is secured through the category of relation, and the Unity is maintained through the fact that there is no difference of substance, or operation, or generally of any substantial predicate. So then, the category of substance preserves the Unity, that of relation brings about the Trinity. Hence only terms belonging to relation may be applied singly to Each. For the Father is not the same as the Son, nor is either of Them the same as the Holy Spirit. Yet Father, Son, and Holy Spirit are each the same God, the same in justice, in goodness, in greatness, and in everything that can be predicated of substance. One must not forget that predicates of relativity do not always involve relation to something other than the subject, as slave involves master, where the two terms are different. For equals are equal, like are like, identicals are identical, each with other, and the relation of Father to Son, and of both to Holy Spirit is a relation of identicals. A relation of this kind is not to be found in created things, but that is because of the difference which we know attaches to transient objects. We must not in speaking of God let imagination lead us astray; we must let the Faculty of pure Knowledge lift us up and teach us to know all things as far as they may be known.[26]
Text from Project Gutenberg, public domain in the United States.