Theologicae regulae

Every distinguishing name, or pronoun, in the masculine or feminine, joined to a personal name, pertains to the person; in the neuter, however, to the essence.

As, the Father is other than the Son, the person of the Father is other than the person of the Son; but not another thing. The Father is not he who is the Son, but he is that which the Son is. Nor is it a wonder if distinguishing names, or pronouns, in the masculine or feminine pertain to the person, in the neuter to the essence, since in natural things names in the masculine or feminine pertain to the thing of a kind, in the neuter to the kind of thing. Adjectives in the masculine and feminine gender, however, are distinguishing by reason of gender; for the masculine or feminine gender is distinguishing: whence also the adjective itself, since by force of discretion it pertains to the thing of a kind, requires a subject which determines the thing of the kind, as, a white horse, a white woman. The adjective, however, in the neuter gender, produces confusion. For the neuter gender is not called a gender by position, but by negation: and so in the neuter gender the adjective, because it is confused, pertains to the kind of the thing, not to the thing of the kind. Whence it is customary to say that in an adjective of neuter gender is understood this subject, "thing," wherefore it is also usually held substantively, as when it is said: The white [thing] runs. Similarly, by "who" in the masculine gender one is accustomed to ask about a person, as when it is said: Who runs? by "who" also in the feminine: by "what" in the neuter, concerning essence. Similarly in divine matters, by reason of discretion, distinguishing names in the masculine and feminine pertain to the person; in the neuter, however, by reason of confusion, to the essence.