Essentialism has been controversial from its beginning. Plato's Socrates already problematizes the concept of the Idea by positing in the Parmenides that if we accept Ideas of such things as Beauty and Justice (every beautiful thing or just action would partake of that Idea in some sense in order to be beautiful or just), we must also accept the "existence of separate forms for hair, mud, and dirt". In biology and other natural sciences, essentialism provided the basis for and rationale of taxonomy at least until the time of Charles Darwin; the precise role and importance of essentialism in biology is still a matter of debate. In gender studies, essentialism (summarized as the basic proposition that men and women are essentially different) continues to be a matter of contention.
French structuralist feminism was often accused of subscribing to an essentialism, which was set in contrast to gender constructionism.
An essence characterizes a substance or a form, in the sense of the Forms or Ideas in Platonic idealism. It is permanent, unalterable, and eternal; and present in every possible world. Classical humanism has an essentialist conception of the human being, which means that it believes in an eternal and unchangeable human nature. The idea of an unchangeable human nature has been criticized by Kierkegaard, Marx, Heidegger, Sartre, and many other existential thinkers.
In Plato’s philosophy (in particular, the Timaeus and the Philebus), things were said to come into being in this world by the action of a demiurge who works to form chaos into ordered entities. Many definitions of essence hark back to the ancient Greek hylomorphic understanding of the formation of the things of this world. According to that account, the structure and real existence of any thing can be understood by analogy to an artifact produced by a craftsman. The craftsman requires hyle (timber or wood) and a model, plan or idea in his own mind according to which the wood is worked to give it the indicated contour or form (morphe). Aristotle was the first to use the terms hyle and morphe. According to his explanation, all entities have two aspects, "matter" and "form". It is the particular form imposed that gives some matter its identity, its quiddity or "whatness" (i.e., its "what it is").
Plato was one of the first essentialists, believing in the concept of ideal forms, an abstract entity of which individual objects are mere facsimiles. To give an example; the ideal form of a circle is a perfect circle, something that is physically impossible to make manifest, yet the circles that we draw and observe clearly have some idea in common — this idea is the ideal form. Plato believed that these ideas are eternal and vastly superior to their manifestations in the world, and that we understand these manifestations in the material world by comparing and relating them to their respective ideal form. Plato's forms are regarded as patriarchs to essentialist dogma simply because they are a case of what is intrinsic and a-contextual of objects — the abstract properties that makes them what they are. For more on forms, read Plato's parable of the cave.
Karl Popper splits the ambiguous term realism into essentialism and realism. He uses essentialism whenever he means the opposite of nominalism, and realism only as opposed to idealism. Popper himself is a realist as opposed to an idealist, but a methodological nominalist as opposed to an essentialist. For example, statements like "a puppy is a young dog" should be read from right to left, as an answer to "What shall we call a young dog"; never from left to right as an answer to "What is a puppy?”