Three Dialogues between Hylas and Philonous in Opposition to Sceptics and Atheists - George Berkeley

Three important concepts discussed in the Three Dialogues are perceptual relativity, the conceivability/master argument ('master argument' was coined by André Gallois), 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.