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 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. Background In 1709, Berkeley published his first major work, An Essay Towards a New Theory of Vision, in which he discussed the limitations of human vision and advanced the theory that the proper objects of sight are not material objects, but light and colour. This foreshadowed his chief philosophical work, A Treatise Concerning the Principles of Human Knowledge (1710), which, after its poor reception, he rewrote into the Three Dialogues (1713). Hylas and Philonous Berkeley's views are represented by Philonous (Greek: "lover of mind"), while Hylas ("hyle", Greek: "matter") embodies the Irish thinker's opponents, in particular John Locke. In The First Dialogue, Hylas expresses his disdain for skepticism, adding that he has heard Philonous to have "maintained the most extravagant opinion that ever entered into the mind of man, to wit, that there is no such thing as material substance in the world." Philonous argues that it is actually Hylas who is the skeptic and that he can prove it. Thus, a philosophical battle of wit begins. Philonous questions Hylas systematically regarding what humans know of the world, first examining secondary qualities, such as heat, to show that such qualities do not exist outside the individual mind. He then moves on to primary qualities such as extension and shape, and likewise argues that they, too, are dependent entirely on one's perception and perspective (e.g., From a distance, a great mountain appears to be small, and the shape of a thing may change dramatically under a microscope: "You may at any time make the experiment, by looking with one eye bare, and with the other through a microscope"). Hylas's view of matter (which has its origin in the Platonic theory of forms , or abstract entities that exist outside of the sensible