The wall which I see is seen as something separate--as apart from my body. This is the external aspect of perception. The colour, the size, and the form of the wall are sensations which are experienced mentally and therefore within me. This is the internal aspect of perception. That a wall is without me I know only by something that happens within me. This may seem paradoxical but the truth is I do not know the externality of the wall but infer it. It is now necessary to attend closely to an examination of the mechanism of what follows. For having surmised the separate and external existence of a wall I have really projected part of my mental experience into the world outside. I have objectified an idea.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 41