Mind and matter are incommensurables. Mind can enter into relations only with something allied to its own subtler nature, not with something wholly dissimilar, as matter is said to be. That which the mind knows must be relevant in relation to the Mind itself. There must be a community of kind between the two, a common identity of substance. The world as known cannot possibly be extra-mental in nature. Hence the characteristics of what the mind knows must be mental--that is, they constitute our ideas.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 162