It's All In Your Head

George Berkeley (From: Wikipedia)

"[George] Berkeley was quite happy to suppose that there was no substantial world apart from the world in our minds. The world was indeed composed of ideas--a position subsequently known as idealism... If there were no 'external' world to serve as cause of our sensations, where would our sensations and our ideas about the world come from? It is God who must provide them, Berkeley argues. 'To be is to be perceived,' he insists, but everything that exists must therefore be perceived, all the time, by God. (It was regarding Berkeley's philosophy that some wit formulated the old gambit, 'If a tree falls in the forest...') Perhaps what is most remarkable is the fact that Berkeley's philosophy, which denies the material existence of the world, held onto the Lockean claim to be merely a matter of 'common sense.' (How often, in philosophy, would the appeal to common sense end up in nonsense.) The English Doctor Johnson thought that Berkeley's idealism was hardly common sense at all, and kicking a stone he commented to a friend, 'Thus I refute him.'" (196) --Robert C. Solomon and Kathleen M. Higgins, A Short History of Philosophy