George Berkeley (pronounced: /ˈbɑrkli/) (12 March 1685 14 January 1753), also known as Bishop Berkeley, was a philosopher. His primary philosophical achievement was the advancement of a theory he called "immaterialism" (later referred to as "subjective idealism" by others). This theory, summed up in his dictum, "Esse est percipi" ("To be is to be perceived"), contends that individuals can only directly know sensations and ideas of objects, not abstractions such as "matter." His most widely-read works are A Treatise Concerning the Principles of Human Knowledge (1710) and Three Dialogues between Hylas and Philonous (1713), in which the characters Philonous and Hylas represent Berkeley himself and his contemporary John Locke. In 1734, he published The Analyst, a critique of the foundations of calculus, which was influential in the development of mathematics. (via Wikipedia)