When we have been looking at a bright light, and then that bright light is shut off, we see darkness. In reality there is no darkness, it only seems so; if there had not been a bright light before there would not be darkness but some light, for it is the contrast that makes it seem dark. Thus the experience which we have in our deep sleep is an experience of a higher and greater kind, and yet it is so fine, so subtle and unusual, because our consciousness is so accustomed to the rigid experiences of the physical world, and when we are in that other state the experience is too fine to perceive and to bring back to the physical world.