Theologicae regulae

In the Father, unity; in the Son, equality; in the Holy Spirit, the connection of unity and equality.

This rule descends from the third, and is as it were an explanation of the third, because the Father is called monad for the reasons stated, namely that he is from no one, and whatever is from him: the Son also is from him through generation, the Holy Spirit through procession: of himself also he begets a monad, that is, a thing of the same nature, and simple. In the Father is specially said to be unity; in the Son, equality. I do not say otherness, lest a diversity of nature be understood there, but rather equality. For in equality is noted both personal plurality and unity of nature; for nothing is equal to itself, nor is anything equal to another which is dissimilar in nature. Now for a threefold reason equality is specially attributed to the Son, although it belongs also to the Holy Spirit: because, first, equality occurs in the Son; because the Holy Spirit also is equal to the Father, this he has from him: equality also, or likeness, is specially observed in that which proceeds from another through generation. Why, moreover, in the Holy Spirit is said to be the connection of unity and equality, is clear from what has already been said.