It stands to reason that the light in the heavens is spiritual and that that light is divine truth when we consider that we also have spiritual light and that we have enlightenment from it to the extent that we participate in intelligence and wisdom on the basis of divine truth. Our spiritual light is the light of our discernment, whose objects are things true that it arranges in order by a process of analysis and forms into relationships, and from which it draws a series of conclusions.
Natural people are unaware that the light that enables us to see such things is a real light because they do not see it with their eyes or notice it with their thought. Still, many people do recognize it and also distinguish it from the natural light in which they find themselves when they are thinking naturally and not spiritually. People are thinking naturally when they are focusing solely on this world and attributing everything to nature. They are thinking spiritually, however, when they focus on heaven and attribute everything to the Divine.
I have often been allowed to perceive that the light that illumines the mind is a true light, quite different from the light that we call natural light. I have also been allowed to see it. I have been gradually elevated into that light inwardly, and as I was raised up, my discernment was enlightened to the extent that I could grasp what I had been unable to grasp before, ultimately things that could in no way be comprehended by thought from natural light. At times I have resented the fact that they were incomprehensible [in natural light] when they were so clearly and plainly perceived in the heavenly light.
Since our mind does have light, we speak of it much as we do of our eyes—for example, that it sees and is in the light when it grasps something, and that it is in darkness and shadows when it does not; and there are many other similar sayings.
from Heaven and Hell, Section 130