George Berkeley also known as Bishop Berkeley, was an Anglo-Irish philosopher whose primary achievement was the advancement of a theory he called "immaterialism" (later referred to as "subjective idealism" by others). This theory contends that individuals can only know directly sensations and ideas of objects, not abstractions such as "matter." The theory also contends that ideas are dependent upon being perceived by minds for their very existence, a belief that became immortalized in the dictum, "Esse est percipi" ("To be is to be perceived"). His most widely-read works are A Treatise Concerning the Principles of Human Knowledge and Three Dialogues between Hylas and Philonous, in which the characters Philonous and Hylas represent Berkeley himself and his older contemporary John Locke.