Experience is a unity and cannot be broken into mind and matter. We cannot possibly separate the world from the mind that knows it. The two are always related. To object that such a relation need not exist outside the act of knowing the world, even though it must exist inside it, is to utter words which dissolve away as soon as their meaning is analysed. For the only world which human beings can ever discuss is one which they can think about and which is therefore an idea for their minds.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 174