The sun shines not there, nor the moon and the stars, nor these lightnings, not to speak of this fire. When He shines, everything shines after Him; All this is illumined by his radiance. – Mundaka Upanishad. Verse 2.2.10.
Sankara’s commentary: How that is the light of all lights is explained. The sun, though enlightening all does not shine in, i.e., does not illumine the Brahman which is his Atman; for, the sun illumines the whole universe other than the Atman with the light of the Brahman, but has not in himself the capacity to illuminate. Similarly neither the moon and the stars nor the lightning shines. How could this be which is in the range of our vision? Why dilate? This universe which shines, shines with the light of Him, the Lord of all, who shines being himself luminously. Just as water and the rest by their contact with fire, heat with the heat of the fire, but not by their own inherent power, so all this universe, the sun and the rest shine with the light of the Brahman. As it is the Brahman alone that thus shines and shines with varying light in its diverse manifestations, itself luminosity, is inferred; for that which is not itself light cannot illumine as we see those pots etc., do not illumine others and that the sun and the rest having light, illumine others.
Aim Hrim Klim