John Calvin Commentary Psalms 36:9

"For with thee is the fountain of life: In thy light shall we see light." — Psalms 36:9 (ASV)

For with thee is the fountain of life. The Psalmist here confirms the doctrine of the preceding verse, the knowledge of which is so profitable that no words can adequately express it. As the ungodly profane even the best of God’s gifts by their wicked abuse of them, unless we observe the distinction which I have stated, it would be better for us to perish a hundred times from hunger than to be fed abundantly by the goodness of God.

The ungodly do not acknowledge that it is in God they live, move, and have their being, but rather imagine that they are sustained by their own power. Accordingly, David, on the contrary, here affirms from the experience of the godly, and so to speak in their name, that the fountain of life is in God. By this he means that there is not a drop of life to be found without him, or which does not flow from his grace.

The metaphor of light, in the last clause of the verse, is tacitly most emphatic, denoting that men are altogether destitute of light, except insofar as the Lord shines upon them.

If this is true of the light of this life, how will we be able to behold the light of the heavenly world, unless the Spirit of God enlightens us? For we must maintain that the measure of understanding with which men are by nature endowed is such that the light shineth in darkness,
but the darkness comprehendeth it not (John 1:5), and that men are enlightened only by a supernatural gift.

But it is the godly alone who perceive that they derive their light from God, and that, without it, they would continue, so to speak, buried and smothered in darkness.