Thinking is possible only where there exists an object about which to think, whether it be a material thing or a mere idea. We cannot think unless we have something in mind. This means in every act of thought there are two elements: the thinking itself and the object or idea thought about. These are so coupled together by the psychological constitution of man that the first cannot exist without the second.
This is equally true of the act of seeing. We cannot see anything unless there is some object, something to be seen. Hence sight depends upon both seeing itself and the object seen. Both are so interrelated that the former could not exist if there were not the other.
These statements may be more easily understood after due reflection, but it will be much more difficult to understand that the contrary ones likewise hold true. That is to say, no object or idea can exist without being thought of, and nothing perceptible can exist without something or someone to see it. In short, the factors which have been coupled together here are mutually dependent.
It is impossible for a thinkable object or idea to exist in a state where thought itself is impossible. It is impossible for a seeable thing to exist in a state where sight is impossible, as in deep sleep. And, since everything material is either thinkable or seeable or both, it follows that the entire material universe has its being in being thought of or perceived. It is only an appearance within the mind of the thinker or dependent upon the perceiver. No idea, no object, could have any conceivable existence if the perceiver himself never had any. Something living and conscious that can think and become aware of them must first exist through their relation to it. They cannot possibly exist in disconnection from a conscious mind.
If we imagine a universal state wherein there was no body present, no mind that could think of anything, perceive it, or be conscious of it, then we are quite unable to put any idea or object or sound or colour into this state.
This is true whether we apply it to mere ideas or to hard and heavy things which we see and feel, such as houses and trees. The point cannot be grasped by the understanding without previous reflection and meditation, for it appears to be contrary to common experience and common sense. In short, matter is a mental sensation and not the cause of a mental sensation.
-- Notebooks Category 21: Mentalism > Chapter 2: The World As Mental > # 154