Monologium, on the Being of God59 Chapter LIX. The Father and the Son and their Spirit exist equally the one in the other.

It is a most interesting consideration that the Father, and the Son, and the Spirit of both, exist in one another with such equality that no one of them surpasses another. For, not only is each in such a way the perfectly supreme Being that, nevertheless, all three at once exist only as one supreme Being, but the same truth is no less capable of proof when each is taken separately.

For the Father exists as a whole in the Son, and in the Spirit common to them; and the Son in the Father, and in the Spirit; and the Spirit in the Father, and in the Son; for the memory of the supreme Being exists, as a whole, in its intelligence and in its love, and the intelligence in its memory and love, and the love in its memory and intelligence. For the supreme Spirit conceives of (intelligit) its memory as a whole, and loves it, and remembers its intelligence as a whole, and loves it as a whole, and remembers its love as a whole, and conceives of it as a whole.

But we mean by the memory, the Father; by the intelligence, the Son; by the love, the Spirit of both. In such equality, therefore, do Father and Son and Spirit embrace one another, and exist in one another, that none of them can be proved to surpass another or to exist without it.