13 With what fountain then will you be overflowed, and whence runs such a torrent of His Pleasure? “For with You,” says he, “is the fountain of Life.” What is the fountain of Life, but Christ? He came to you in the flesh, that He might bedew your thirsty lips: He will satisfy you trusting, who bedewed you thirsting. “For with You is the fountain of Life; in Your Light shall we see light”. Here a fountain is one thing, light another: there not so. For that which is the Fountain, the same is also Light: and whatever you will you call It, for It is not what you call It: for you can not find a fit name: for It remains not in one name.
If you should say, that It is Light only, it would be said to you, Then without cause am I told to hunger and thirst, for who is there that eats light? It is said to me plainly, directly, “Blessed are the pure in heart: for they shall see God.” If It is Light, my eyes must I prepare. Prepare also lips; for That which is Light is also a Fountain: a Fountain, because It satisfies the thirsty: Light, because It enlightens the blind. Here sometimes, light is in one place, a fountain in another. For sometimes fountains run even in darkness; and sometimes in the desert you suffer the sun, findest no fountain: here then can these two be separated: there you shall not be wearied, for there is a Fountain; there you shall not be darkened, for there is Light.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)