Does the world exist outside of and separately from the mind that knows it? This is quite a different question from that which deals with its relation to the body. Nobody could dispute its outsideness and separateness then. But the question we are really asking is not so simple. For the light-born image of the world which forms itself on the retina of the eye, the awareness of things touched, smelled, or tasted, is all that the mind actually knows. It cannot speak and has no right to speak of any world which possibly lies beyond its frontiers.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 21