5 That unfailing Light, the Light of wisdom, speaking through the cloud of the flesh, says to men, “I am the light of the world; he that follows me shall not walk in darkness, but shall have the light of life.” How He has withdrawn you from the eyes of the flesh, and recalled you to the eyes of the heart! For it is not enough to say, “Whoso follows me shall not walk in darkness, but shall have light;” He added too, “of life;” even as it was there said, “For with You is the fountain of life.” See thus, my brethren, how the words of the Lord agree with the truth of that psalm: both there, the light is put with the fountain of life, and by the Lord it is said, “light of life.” But for bodily use, light and fountain are different things: our mouths seek a fountain, our eyes light; when we thirst we seek a fountain, when we are in darkness we seek light; and if we chance to thirst in the night, we kindle a light to come to a fountain. Not so with God: light and fountain are the same thing: He who shines for you that you may see, the same flows for you that you may drink.
Source: Tractates on the Gospel of John (New Advent)