Theologicae regulae

Every name that is said of God is either essential, or co-essential, or personal, or co-personal, or partly personal, partly essential.

Essential names are called all those names which suppose the essence, and add to it. These are, as it were, mathematical, either predicating the essence, that is, principal names, as Deity, Divinity; or naming the person, as this name, God: for, as was said above, this name, God, predicates Deity, and supposes person: sometimes, however, it supposes deity; as, God is the Father, and the Son, and the Holy Spirit. Nor is it a wonder: since Deity itself is a person. For in natural things, where the thing of the nature is one thing, the nature of the thing another, although it properly has to suppose the thing of the nature, it sometimes improperly supposes the nature of the thing. For example, this name, man, properly supposes a man; as when it is said: A man runs; improperly, however, it supposes the species, as when it is said: Man is a species.

Co-essential names are called those which signify, or predicate, circumstances of substance, that is, relations attached to substance, and name persons, as, principle, Lord, creator, author. Now of these names, some signify eternal relations, some relations from time, as these names, principle, Lord; for from eternity he was Lord, or principle. Temporal relations, however, are signified by these: creator, author, maker; for in time God began to be creator, or author. Now these names are sometimes set down substantively, with the article understood, as the Father, and the Son, and the Holy Spirit are principle, or Lord; and then it is understood that the essence is predicated, and the relation co-predicated; sometimes they are held adjectivally, without the article understood, as when it is said, the Father is lord, and principle of all creatures: and then it is understood that the relation is predicated, and the essence co-predicated. Personal names are called those which suppose and predicate personal properties, as, paternity, filiation, procession; or which predicate personal properties, and name persons, as Father, Son, Holy Spirit, begotten, begetting, proceeding. Co-personal names are called those which predicate distinctions attached to personal properties, as, distinct, differing, other. Partly personal, partly essential names are called those which partly note the person, partly the essence, as this name, person, and this name, Trinity. Person is called as it were "one through itself." By this, then, that unity is understood there, an understanding of essence is had; but by the fact that it is said "through itself," an understanding of personal distinction is had. Whence the Father is called person, as it were, one through itself, that is, distinct in such a way that it is one. Likewise the Son, likewise the Holy Spirit. Whence also they are called persons, as it were, one through themselves, that is, distinct in such a way that they are one. Likewise Trinity, as it were, three, one, so that by this that III are understood, there is knowledge of personal distinction; by "one," of the essence.