Works of St. Anselm, tr. by Sidney Norton Deane, [1903], at sacred-texts.com
How one is the essence of the other.
HENCE, even if one is called the essence of the other, there is no departure from truth; but the supreme p. 106 simplicity and unity of their common nature is thus honored. For, not as one conceives of a man’s wisdom, through which man is wise, though he cannot be wise through himself, can we thus understand the statement that the Father is essence of the Son, and the Son the essence of the Father. We cannot understand that the Son is existent through the Father, and the Father through the Son, as if the one could not be existent except through the other, just as a man cannot be wise except through wisdom.
For, as the supreme Wisdom is ever wise through itself, so the supreme Essence ever exists through itself. But, the perfectly supreme Essence is the Father, and the perfectly supreme Essence is the Son. Hence, the perfect Father and the perfect Son exist, each through himself, just as each is wise through himself.
For the Son is not the less perfect essence or wisdom because he is an essence born of the essence of the Father, and a wisdom born of the wisdom of the Father; but he would be a less perfect essence or wisdom if he did not exist through himself, and were not wise through himself.
For, there is no inconsistency between the subsistence of the Son through himself, and his deriving existence from his Father. For, as the Father has essence, and wisdom, and life in himself; so that not through another’s, but through his own, essence he exists; through his own wisdom he is wise; through his own life he lives; so, by generation, he grants to his Son the possession of essence, and wisdom, and life in himself, so that not through an extraneous essence, wisdom, and life, but through his own, he subsists, is wise, and lives; otherwise, the existence of p. 107 Father and Son will not be the same, nor will the Son be equal to the Father. But it has already been clearly proved how false this supposition is.
Hence, there is no inconsistency between the subsistence of the Son through himself, and his deriving existence from the Father, since he must have from the Father this very power of subsisting through himself. For, if a wise man should teach me his wisdom, which I formerly lacked, he might without impropriety be said to teach me by this very wisdom of his. But, although my wisdom would derive its existence and the fact of its being from his wisdom, yet when my wisdom once existed, it would be no other essence than its own, nor would it be wise except through itself.
Much more, then, the eternal Father’s eternal Son, who so derives existence from the Father that they are not two essences, subsists, is wise, and lives through himself. Hence, it is inconceivable that the Father should be the essence of the Son, or the Son the essence of the Father, on the ground that the one could not subsist through itself, but must subsist through the other. But in order to indicate how they share in an essence supremely simple and supremely one, it may consistently be said, and conceived, that the one is so identical with the other that the one possesses the essence of the other.
On these grounds, then, since there is obviously no difference between possessing an essence and being an essence, just as the one possesses the essence of the other, so the one is the essence of the other, that is, the one has the same existence with the other.