essence, in philosophy, the nature of a thing. Aristotle maintained that there is a distinction between the form of a thing—its intelligible, verbally formulable character—and the essence of a thing, i.e., what it is in itself, which is not common to anything else. The essence of a thing is what is formulated as a universal in the mind and in language. St. Thomas Aquinas distinguished between the essence of a thing and the fact of its being, or its existence. In modern existentialist thought Jean-Paul Sartre made use of Aquinas's distinction between essence and existence but reversed them by insisting that existence precedes essence. By this he asserted that people do not have predetermined natures; what a person is follows from the choices he or she makes.