Reply Obj. 1: This argument is true of real union. That is necessary to pleasure as being its cause; desire implies the real absence of the beloved: but love remains whether the beloved be absent or present.
Reply Obj. 2: Union has a threefold relation to love. There is union which causes love; and this is substantial union, as regards the love with which one loves oneself; while as regards the love wherewith one loves other things, it is the union of likeness, as stated above (Q. 27, A. 3). There is also a union which is essentially love itself. This union is according to a bond of affection, and is likened to substantial union, inasmuch as the lover stands to the object of his love, as to himself, if it be love of friendship; as to something belonging to himself, if it be love of concupiscence. Again there is a union, which is the effect of love. This is real union, which the lover seeks with the object of his love. Moreover this union is in keeping with the demands of love: for as the Philosopher relates (Polit. ii, 1), "Aristophanes stated that lovers would wish to be united both into one," but since "this would result in either one or both being destroyed," they seek a suitable and becoming union—to live together, speak together, and be united together in other like things.
Reply Obj. 3: Knowledge is perfected by the thing known being united, through its likeness, to the knower. But the effect of love is that the thing itself which is loved, is, in a way, united to the lover, as stated above. Consequently the union caused by love is closer than that which is caused by knowledge. ________________________
SECOND ARTICLE [I-II, Q. 28, Art. 2]
Whether Mutual Indwelling Is an Effect of Love?
Objection 1: It would seem that love does not cause mutual indwelling, so that the lover be in the beloved and vice versa. For that which is in another is contained in it. But the same cannot be container and contents. Therefore love cannot cause mutual indwelling, so that the lover be in the beloved and vice versa.
Obj. 2: Further, nothing can penetrate within a whole, except by means of a division of the whole. But it is the function of the reason, not of the appetite where love resides, to divide things that are really united. Therefore mutual indwelling is not an effect of love.
Obj. 3: Further, if love involves the lover being in the beloved and vice versa, it follows that the beloved is united to the lover, in the same way as the lover is united to the beloved. But the union itself is love, as stated above (A. 1). Therefore it follows that the lover is always loved by the object of his love; which is evidently false. Therefore mutual indwelling is not an effect of love.
On the contrary, It is written (1 John 4:16): "He that abideth in charity abideth in God, and God in him." Now charity is the love of God. Therefore, for the same reason, every love makes the beloved to be in the lover, and vice versa.
I answer that, This effect of mutual indwelling may be understood as referring both to the apprehensive and to the appetitive power. Because, as to the apprehensive power, the beloved is said to be in the lover, inasmuch as the beloved abides in the apprehension of the lover, according to Phil. 1:7, "For that I have you in my heart": while the lover is said to be in the beloved, according to apprehension, inasmuch as the lover is not satisfied with a superficial apprehension of the beloved, but strives to gain an intimate knowledge of everything pertaining to the beloved, so as to penetrate into his very soul. Thus it is written concerning the Holy Ghost, Who is God's Love, that He "searcheth all things, yea the deep things of God" (1 Cor. 2:10).