In order to understand this we must go back to what we learnt in the course of our optical studies as to the two forms of vision arising from the activity of the eye's inner light - the dream-vision and the seeing of after-images. Of these two, seeing in dream is in a certain sense the purer form of inner seeing in that it arises without any outer stimulus exercised upon the physical organ of sight. On the other hand, it lacks that objective conformity to law characteristic of the after-images which mirror the order of the external world. There is an arbitrary, enigmatic element in dream-pictures, and their logic often seems to run counter to that of waking consciousness. A further characteristic of dream-perception is that we are tied to the level of consciousness prevailing in the dream. While we are dreaming we cannot awaken to the extent of being able to make the pictures the object of conscious observation.