Three Dialogues between Hylas and Philonous

Berkeley's restatement of his immaterialism as three conversations between Hylas, who defends matter, and Philonous, the 'lover of mind', written after the Principles had found few readers. In the First Dialogue Philonous argues that heat, colour, taste and sound exist only in being perceived, then turns the same arguments on extension, shape and motion, so that Locke's line between primary and secondary qualities collapses. Hylas tries to rescue matter as a hidden support of qualities, or as something our ideas copy, and fails each time. The Second Dialogue presents God as the mind in which sensible things exist when no human perceives them, and rejects matter as cause, instrument or occasion of our ideas. The Third Dialogue takes objections in turn: that this is scepticism, that it cannot tell real things from dreams, that it cannot explain the creation, and that it defies common sense. Philonous claims to side with ordinary people, who trust their senses, against philosophers who doubt them.

It is the liveliest way into idealism, and its arguments about perception and qualities are still used in teaching because they are hard to answer cleanly.