In St. Thomas Aquinas’ theology of friendship, one of the most interested phrases he uses is “mutual indwelling.” In the clarity of his thought, he uses very technical and precise language in order to describe the union which is an effect of love. This theology encompasses many different kinds of relationships such as natural friendships, marriage, and our relationship with God.
Natural friendships involve a level of affinity which penetrates deeper than mere sentiment. Empirical sciences recognize that we are fundamentally social creatures, and that our relationships have a tendency to shape and mold who we are. Popularly, this gets expressed in ideas such as “peer pressure” which express the role of the company we keep. St. Thomas takes this a step further. Our fraternal love for others draws us into a deeper form of union. We internalize who our friends are and this has an effect on our personality.
Marriage is a higher form of friendship, one in which man and woman are joined in a deeper communion than merely natural affinity. It resembles natural friendships, but involves a deeper interpenetration. This union between husband and wife involves a progressive intimacy and a real growth. The psychological, emotional, and physical dimensions of marital love involve a real maturation towards greater unity between spouses. As married couples grow in the mutual experience of love, they take in the other and are joined a very real and tangible way.
So what does this mean in our daily lives? I would argue that this kind of thinking opens us to the mystery of who we are. When shaped by such ideas, our friendships and relationships take on a deeper significance. This teaching helps us to nurture our friendships because in our friends who truly have what Psalm 55 calls an “other self.” It opens us to reality that we are creatures made to be in relationship with God and neighbor.