It is absurd even to suggest that there is an external world wholly outside of one's consciousness and wholly independent of it. One knows only certain changes of mental awareness, never of externals. The mind can only know its changes of individual consciousness. All its observations, each of its inferences, everything it knows--these lie enclosed within that consciousness and are never beyond it.
One's knowledge of anything whatsoever is simply one's thought of it. This is not to be confused with one's right thought of it. It is a conscious mental state, and even other persons are but appearances within this state, creatures in the cosmic dream. To follow this line of reflection to its inevitable end demands courage and candour of the highest kind, for it demands as ultimate conclusion the principle that knowledge being but ideas in the mind, the whole universe is nothing but an immense idea within one's own mind. For the very nature of knowledge is thus internal, and hence the individual mind cannot know any reality external to itself. It believes that it observes a world without when it only observes its own mental pictures of that world.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 108
-- Perspectives > Chapter 21: Mentalism > # 48