2 Accordingly, the Lord cries aloud to us: for, “He stood and cried out, if any man thirst, let him come unto me, and drink. He that believes in me, as the Scripture says, out of his belly shall flow rivers of living water.” We are not obliged to delay to inquire what this meant, since the evangelist has explained it. For why the Lord said, “If any man thirst, let him come unto me, and drink;” and, “He that believes in me, out of his belly shall flow rivers of living water;” the evangelist has subsequently explained, saying: “But this spoke He of the Spirit which they that believe in Him should receive.
For the Spirit was not yet given, because Jesus was not yet glorified.” There is therefore an inner thirst and an inner belly, because there is an inner man. And that inner man is indeed invisible, but the outer man is visible; but yet better is the inner than the outer. And this which is not seen is the more loved; for it is certain that the inner man is loved more than the outer. How is this certain? Let every man prove it in himself. For although they who live ill may surrender their minds to the body, yet they do wish to live, and to live is the property of the mind only; and they who rule, manifest themselves more than those things that are ruled.
Now it is minds that rule, bodies are ruled. Every man rejoices in pleasure, and receives pleasure by the body: but separate the mind from it, and nothing remains in the body to rejoice; and if there is joy of the body, it is the mind that rejoices. If it has joy of its dwelling, ought it not to have joy of itself? And if the mind has whereof it may have delight outside itself, does it remain without delights within? It is quite certain that a man loves his soul more than his body.
But further, a man loves the soul even in another man more than the body. What is it that is loved in a friend, where the love is the purer and more sincere? What in the friend is loved— the mind, or the body? If fidelity is loved, the mind is loved; if benevolence is loved, the mind is the seat of benevolence: if this is what you love in another, that he too loves you, it is the mind you love, because it is not the flesh, but the mind that loves. For therefore you love, because he loves you: ask why he loves you, and then see what it is you love. Consequently, it is more loved, and yet is not seen.
Source: Tractates on the Gospel of John (New Advent)