Herbert Spencer admitted the truth of mentalism in his Principles of Psychology (Vol. 2, Part 7). He admitted that the world we know is mentally constructed and mentally existent. Having got so far, he then fell into error, for he said that our experience of the resistance which objects in the world offer us proves that they also exist independently of and outside the mind. What was Spencer's mistake "of all of the objective idealists"? It was the failure to penetrate sufficiently far into the meaning of these two words: "independent" and "outside." How can the world have an independent existence when it has no significance for us before we actually experience it? It must touch our body or affect our senses before its existence comes to have any meaning at all for us. When this happens we have the feelings or thoughts which science calls sensations. Whether they are feelings of hardness, resistance, or weight, thoughts of redness, fragrance, or noise, they are still nothing else than our feelings and thoughts. Where is the independence here? The objects in the world are only objects of our consciousness. They may be independent in relation to our body but they are not independent in relation to our senses and hence to our mind. The sensations of resistance and hardness are no less mental ultimately than are any of the other sensations. Again, where is the outsideness here? Does the world really stand outside the mind that knows it? It is only at the cost of self-contradiction that we can answer that it does so stand. For whatever is in consciousness, whatever is mental, can be explained by the mind alone. It is the mind's own activity which makes resistance as it makes smells, sounds, and sights. Furthermore it is this same activity which creates the space-relationships between objects and hence the thought of their outsideness.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 47