The Dream Argument for Scepticism about the External World:
(1) Subjective experience is either a dream or reality.
(2) If subjective experience is a dream, then it is caused by the mind.
(3) If subjective experience is reality, then it is caused by the world.
(4) There is no difference between the experience of a dream and the experience of reality.
Hence, there is no way of knowing if there is a world, or if it is all in the mind.