Works of St. Anselm, tr. by Sidney Norton Deane, [1903], at sacred-texts.com
Consideration of the common attributes of both and the individual properties of each.
NOW that so many and so important properties of each have been discovered, whereby a strange plurality, as ineffable as it is inevitable, is proved to exist in the supreme unity, I think it most interesting to reflect, again and again, upon so unfathomable a mystery.
For observe: although it is so impossible that he who begets, and he who is begotten, are the same, and that parent and offspring are the same ‑‑so impossible that necessarily one must be the progenitor and the other the begotten, and one the Father, the other the Son; yet, here it is so necessary that he who begets and he who is begotten shall be the same, and also that parent and offspring shall be the same, that the progenitor cannot be any other than what the begotten is, nor the Father any other than the Son.
And although the one is one, and the other another, so that it is altogether evident that they are two; yet that which the one and the other are is in such a way one and the same, that it is a most obscure mystery why they are two. For, in such a way is one the Father and the other the Son, that when I speak of both I perceive that I have spoken of two; p. 105 and yet so identical is that which both Father and Son are, that I do not understand why they are two of whom I have spoken.
For, although the Father separately is the perfectly supreme Spirit, and the Son separately is the perfectly supreme Spirit, yet, so are the Spirit‑Father and the Spirit‑Son one and the same being, that the Father and the Son are not two spirits, but one Spirit. For, just as to separate properties of separate beings, plurality is not attributed, since they are not properties of two things, so, what is common to both preserves an indivisible unity, although it belongs, as a whole, to them taken separately.
For, as there are not two fathers or two sons, but one Father and one Son, since separate properties belong to separate beings, so there are not two spirits, but one Spirit; although it belongs both to the Father, taken separately, and to the Son, taken separately, to be the perfect Spirit. For so opposite are their relations, that the one never assumes the property of the other; so harmonious are they in nature, that the one ever contains the essence of the other. For they are so diverse by virtue of the fact that the one is the Father and the other the Son, that the Father is never called the Son, nor the Son the Father; and they are so identical, by virtue of their substance, that the essence of the Son is ever in the Father, and the essence of the Father in the Son.