Through reflection or introspection, is it possible to attempt to know if a sound, shape, movement, or color can exist unperceived by a mind? This book largely seeks to refute the claims made by Berkeley's contemporary John Locke about the nature of human perception. Both Locke and Berkeley agreed that there was an outside world, and it was this world which caused the ideas one has within one's mind. Berkeley sought to prove that the outside world was also composed solely of ideas.