perfect and that its observed imperfections came from man's limited perception of it. For Plato, there were two realities: the "essential" or ideal and the "perceived". Aristotle (384–322 BC) applied the term essence to that which things in a category have in common and without which they cannot be members of that category (for example, rationality is the essence of man; without rationality a creature cannot be a man). In his critique of Aristotle's philosophy, Bertrand Russell said that his concept of essence transferred to metaphysics what was only a verbal convenience and that it confused the properties of language with the properties of the world. In fact, a thing's "essence" consisted in those defining properties without which we could not use the name for it. Although the concept of essence was "hopelessly muddled" it became part of every philosophy until modern times. The Egyptian-born philosopher Plotinus (204–270 AD) brought idealism to the Roman Empire as Neoplatonism, and with it the concept that not only do all existents emanate from a "primary essence" but that the mind plays an active role in shaping or ordering the objects of perception, rather than passively receiving empirical data. Despite the metaphysical basis for the term, academics in science, aesthetics, heuristics, psychology, and gender-based sociological studies have advanced their causes under the banner of essentialism. Possibly the clearest definition for this philosophy was offered by gay/lesbian rights advocate Diana Fuss, who wrote: "Essentialism is most commonly understood as a belief in the real, true essence of things, the invariable and fixed properties which define the 'whatness' of a given entity." Metaphysical essentialism stands diametrically opposed to existential realism in that finite existence is only differentiated appearance, whereas "ultimate reality" is held to be absolute essence. In psychology There is a difference between metaphysical essentialism (see above) and psychological essentialism, the latter referring not to an actual claim about the world but a claim about a way of representing entities in cognitions (Medin, 1989). Influential in this area is Susan Gelman, who has outlined many domains in which children and adults construe classes of entities, particularly biological entities, in essentialist terms—i.e., as if they had an immutable underlying essence which can be used to predict unobserved similarities between members of that class. (Toosi & Ambady, 2011). This causal relationship is unidirectional; an observable feature of an entity does not define the underlying essence (Dar-Nimrod &