The union of lover and beloved is twofold. The first is real union; for instance, when the beloved is present with the lover. The second is union of affection: and this union must be considered in relation to the preceding apprehension; since movement of the appetite follows apprehension. Now love being twofold, viz. love of concupiscence and love of friendship; each of these arises from a kind of apprehension of the oneness of the thing loved with the lover. For when we love a thing, by desiring it, we apprehend it as belonging to our well-being. In like manner when a man loves another with the love of friendship, he wills good to him, just as he wills good to himself: wherefore he apprehends him as his other self, in so far, to wit, as he wills good to him as to himself. Hence a friend is called a man's "other self" (Ethic. ix, 4), and Augustine says (Confess. iv, 6), "Well did one say to his friend: Thou half of my soul."
The first of these unions is caused "effectively" by love; because love moves man to desire and seek the presence of the beloved, as of something suitable and belonging to him. The second union is caused "formally" by love; because love itself is this union or bond. In this sense Augustine says (De Trin. viii, 10) that "love is a vital principle uniting, or seeking to unite two together, the lover, to wit, and the beloved." For in describing it as "uniting" he refers to the union of affection, without which there is nolove: and in saying that "it seeks to unite," he refers to real union.