14 The Lord speaks somewhat more clearly of that living water. Now the woman had said, “Are you greater than our father Jacob, who gave us the well, and drank of it himself, his children, and his cattle?” You can not give me of the living water of this well, because you have nothing to draw with: perhaps you promise another fountain? Can you be better than our father, who dug this well, and used it himself, and his? Let the Lord, then, declare what He called living water. “Jesus answered and said unto her, Every one that drinks of this water shall thirst again: but he that drinks of the water that I shall give him, shall not thirst forever; but the water which I shall give him will become in him a fountain of water, springing up into everlasting life.” The Lord has spoken more openly: “It shall become in him a fountain of water, springing up into everlasting life. He that drinks of this water shall not thirst forever.” What more evident than that it was not visible, but invisible water, that He was promising? What more evident than that He was speaking, not in a carnal, but in a spiritual sense?
Source: Tractates on the Gospel of John (New Advent)