Thomas Aquinas, Compendium of Theology

Since the object known is in the knower and the beloved is in the lover, the different ways of existing in something must be considered in the two cases before us. The act of understanding takes place by a certain assimilation of the knower to the object known; and so the object known must be in the knower in the sense that a likeness of it is present in him. But the act of loving takes place through a sort of impulse engendered in the lover by the beloved: the beloved draws the lover to himself. Accordingly the act of loving reaches its perfection, not in a likeness of the beloved, in the way that the act of understanding reaches perfection in a likeness of the object understood, but in a drawing of the lover to the beloved in person.

The transferring of the likeness of the original is effected by univocal generation whereby, among living beings, the begetter is called father, and the begotten is called son. Among such beings, moreover, the first motion occurs conformably to the species. Therefore, as within the Godhead the way whereby God is in God as the known in the knower, is expressed by what we call "Son," who is the Word of God, so the way by which God is in God as the beloved is in the lover is brought out by acknowledging in God a Spirit, who is the love of God. And so, according to the rule of Catholic faith, we are directed to believe in the Spirit.