We accept the first and chief suggestion of our senses without inquiry, the suggestion that we are dealing with a world totally outside us. It is an error which arises because we do not possess a deep enough understanding of ourselves. But this ignorance arises in its own turn because we do not penetrate deeply enough into our understanding of the world. Hence, the way out of it involves a twofold inquiry: into both self and not-self.
-- Notebooks Category 21: Mentalism > Chapter 1: The Sensed World > # 4