In philosophy, essence is the attribute or set of attributes that make an entity or substance what it fundamentally is, and which it has by necessity, and without which it loses its identity. Essence is contrasted with accident: a property that the entity or substance has contingency, without which the substance can still retain its identity. The concept originates with Aristotle, who used the Greek expression to ti ên einai (τὸ τί ἦν εἶναι, literally meaning "the what it was to be" and corresponding to the scholastic term quiddity) or sometimes the shorter phrase to ti esti (τὸ τί ἐστι, literally meaning "the what it is" and corresponding to the scholastic term haecceity) for the same idea. This phrase presented such difficulties for its Latin translators that they coined the word essentia (English "essence") to represent the whole expression. For Aristotle and his scholastic followers, the notion of essence is closely linked to that of definition (ὁρισμός horismos).In the history of western thought, essence has often served as a vehicle for doctrines that tend to individuate different forms of existence as well as different identity conditions for objects and properties; in this eminently logical meaning, the concept has given a strong theoretical and common-sense basis to the whole family of logical theories based on the "possible worlds" analogy set up by Leibniz and developed in the intensional logic from Carnap to Kripke, which was later challenged by "extensionalist" philosophers such as Quine.