Dialogues between Hylas and Philonous

Three Dialogues between Hylas and Philonous, or simply Three Dialogues, is a 1713 book on metaphysics and idealism written by George Berkeley. Taking the form of a dialogue, the book was written as a response to the criticism Berkeley experienced after publishing A Treatise Concerning the Principles of Human Knowledge. Three important concepts discussed in the Three Dialogues are perceptual relativity, the conceivability/master argument[a] and Berkeley's phenomenalism.
Perceptual relativity argues that the same object can appear to have different characteristics (e. g. shape) depending on the observer's perspective. Since objective features of objects cannot change without an inherent change in the object itself, shape must not be an objective feature.