Hilary of Poitiers

(vii. de Trin) But the Father is in the Son, and the Son in the Father, not by a conjunction of two harmonizing essences3, nor by a nature grafted into a more capacious substance as in material bodies, in which it is impossible that what is within can be made external to that which contains it; but by the birth of a nature which is life from life; forasmuch as from God nothing but God can be born.