In what are probably his two most famous works, the Principles of Human Knowledge (P) and Three Dialogues between Hylas and Philonous (D), George Berkeley argued for his most infamous doctrine--the idea that the objects of everyday experience are in fact ideas in the mind, not material objects that exist independently of their being perceived. Berkeley's theory--known as Idealism--seems obviously absurd (insane, frankly) but is notoriously resistant to refutation. It belongs to a long tradition in philosophy in which no idea is too crazy to put forward in an effort to achieve one's philosophical goals. In this way Berkeley's Idealism belongs in the same corner of the attic as Parmenides' monism, Plato's forms, Pyrrho's universal scepticism, and Leibniz's monads. The sorts of things you dust off and look at with great interest once in a while, but that don't really have an impact on the way you get around in the world.