Works of St. Anselm, tr. by Sidney Norton Deane, [1903], at sacred-texts.com
How among them there is only one Son of one Father, that is, one Word, and that from the Father alone.
ON these grounds, therefore, there apparently are in that Being, not only many fathers and sons and beings proceeding from it, but other necessary attributes as well; or else Father and Son and their Spirit, of whom it is already certain that they truly exist, are not three expressive beings, although each taken separately is expressive, nor are there more beings than one expressed, when each one expresses himself and the other two.
For, just as it is an inherent property of the supreme Wisdom to know and conceive, so it is assuredly natural to eternal and immutable knowledge and intelligence ever to regard as present what it knows and conceives of. For, to such a supreme Spirit expressing and beholding through conception, as it were, are the same, just as the expression of our human mind is nothing but the intuition of the thinker.
But reasons already considered have shown most convincingly that whatever is essentially inherent in the supreme Nature is perfectly consistent with the nature of the Father and the Son and their Spirit taken separately; and that, nevertheless, this, if attributed to the three at once, does not admit of plurality. Now, it is established that as knowledge and intelligence are attributes of his being, so his knowing and conceiving is nothing else than his expression, that is, his ever beholding as present what he knows and conceives of. Necessarily, therefore, just as the Father separately, and the Son separately, and their p. 124 Spirit separately, is a knowing and conceiving being, and yet the three at once are not more knowing and conceiving beings than one, but one knowing and one conceiving being: so, each taken separately is expressive, and yet there are not three expressive beings at once, but one expressive being.
Hence, this fact may also be clearly recognised, that when these three are expressed, either by themselves or by another, there are not more beings than one expressed. For what is therein expressed except their being? If, then, that Being is one and only one, then what is expressed is one and only one; therefore, if it is in them one and only one which expresses, and one which is expressed ‑‑for it is one wisdom which expresses and one substance which is expressed ‑‑it follows that there are not more words than one, but one alone. Hence, although each one expresses himself and all express one another, nevertheless there cannot be in the supreme Being another Word than that already shown to be born of him whose is the Word, so that it may be called his true image and his Son.
And in this truth I find a strange and inexplicable factor. For observe: although it is manifest that each one, that is, Father and Son, and the Spirit of Father and Son equally expresses himself and both the others, and that there is one Word alone among them; yet it appears that this Word itself can in no wise be called the Word of all three, but only of one.
For it has been proved that it is the image and Son of him whose Word it is. And it is plain that it cannot properly be called either the image or son of itself, or of the Spirit proceeding from it. For, neither of itself nor of a being proceeding from it, is it born, p. 125 nor does it in its existence imitate itself or a being proceeding from itself. For it does not imitate itself, or take on a like existence to itself, because imitation and likeness are impossible where only one being is concerned, but require plurality of beings; while it does not imitate the spirit, nor does it exist in his likeness, because it has not its existence from that Spirit, but the Spirit from it. It is to be concluded that this sole Word corresponds to him alone, from whom it has existence by generation, and after whose complete likeness it exists.
One Father, then, and not more than one Father; one Son, and not more than one Son; one Spirit proceeding from them, and not more than one such Spirit, exist in the supreme Being. And, although there are three, so that the Father is never the Son or the Spirit proceeding from them, nor the Son at any time the Father or the Spirit, nor the Spirit of Father and Son ever the Father or the Son; and each separately is so perfect that he is self‑sufficient, needing neither of the others; yet what they are is in such a way one that just as it cannot be attributed to them taken separately as plural, so, neither can it be attributed to them as plural, when the three are taken at once. And though each one expresses himself and all express one another, yet there are not among them more words than one, but one; and this Word corresponds not to each separately, nor to all together, but to one alone.