if I allow myself to talk the language of common sense, the furniture of my room, the trees waving in the wind, houses, clouds, blue sky, and sun. All these common sense imagines to be outside me. All these I believe to be causally connected with physical objects which are outside me, but as soon as I realize that the physical objects must differ in important ways from what I directly experience, and as soon as I take account of the causal trains that proceed from the physical object to my brain before my sensations occur, I see that from the point of view of physical causation the immediately experienced objects of sense are in my brain and not in the outer world.
Source: Bertrand Russell : Mind and Matter (1950?)
More info.:https://russell-j.com/beginner/19501110_Mind-Matter140.HTM