George Berkeley \bark'-lee\ (1685-1753) was an influential Irish philosopher whose primary philosophical achievement is the advancement of what has come to be called subjective idealism, summed up in his catchphrase, "To be is to be perceived." He wrote a number of works, the most widely-read of which are his Treatise Concerning the Principles of Human Knowledge (1710) and Three Dialogues between Hylas and Philonous (1713) (Philonous, the "lover of the mind," representing Berkeley himself).
Discussion (to be rendered into tighter prose?):
You see a redwood tree. Ha! It's only there while you're looking at it. It was only an image in the mind of God, which the Almighty let you hallucinate on.
In response to the old riddle, "If a tree falls in the forest and no one heard it, did it make a sound?," Berkeley would reply that if no one were there, the tree wouldn't be there.
This is eerily similar to recent theoretical physics notion that mass does not exist.
As Bob Dylan sang about dreams, "It's all in your head."