The theory that we perceive the outer world through a sensing process which results in a picture arising in the brain, or on the brain's surface as it does on the eye's surface, still leaves unexplained how we are able to perceive this picture itself. The brain cannot see it for it cannot see colours--only the eye can do that. Nor can the brain feel it, for then it would have to touch it, which would be impossible in the case of large pictures of outer objects larger than itself. Nor can the picture look at, feel, or experience itself. The gap in this theory cannot be crossed. Only by reversing this theory and acknowledging that our awareness of the world really comes to us from within, that by a trick of the mind it only seems to come from without, can the correct and true explanation be found.
-- Notebooks Category 21: Mentalism > Chapter 1 : The Sensed World > # 139
