The point in consciousness where the mind projects its thoughts has been called by the ancients "the cave" or "the cave of the heart." This is because to the outside observer there is nothing but darkness in it and therefore the cave hides whatever it may contain. When, by an inward re-orientation of attention, we trace thoughts, whether of external things or internal fancies, to their hidden origin and penetrate the dark shroud around it, we penetrate into Mind, the divine Overself. We cannot help remembering Gray's apposite lines: "Full many a gem of purest ray serene, The dark unfathomed caves of ocean bear."
-- Notebooks Category 22: Inspiration and the Overself > Chapter 3: The Overself's Presence > # 244