In Three Dialogues Between Hylas and Philonous, George Berkeley defends the view that matter does not exist, that the universe contains minds or spirits but no realm of atoms and molecules. Berkeley argues that things that are normally considered material objects—stones, trees, shoes, apples—have no existence outside the minds and experiences of conscious beings. Like an object in a dream, a stone has no existence outside consciousness. If all conscious beings were to stop perceiving some sensible object—the moon, for example—that object would cease to exist. Although Berkeley’s book owes its philosophical greatness to the many important arguments that are presented in support of the main thesis, the work is also notable for the simplicity and clarity with which the ideas are conveyed. The ideas are presented in the form of a dialogue between Hylas, a materialist, and Philonous, the representative of Berkeley’s idealism.