Essentialism is the view that objects have a set of attributes that are necessary to their identity. In early Western thought, Plato's idealism held that all things have such an "essence"—an "idea" or "form". In Categories, Aristotle similarly proposed that all objects have a substance that, as George Lakoff put it, "make the thing what it is, and without which it would be not that kind of thing". The contrary view—non-essentialism—denies the need to posit such an "essence'".

Essentialism has been controversial from its beginning. Plato, in the Parmenides dialogue, depicts Socrates questioning the notion, suggesting that if we accept the idea that every beautiful thing or just action partakes of an essence to be beautiful or just, we must also accept the "existence of separate essences for hair, mud, and dirt". In biology and other natural sciences, essentialism provided the rationale for taxonomy at least until the time of Charles Darwin; the role and importance of essentialism in biology is still a matter of debate.

Historically, beliefs which posit that social identities such as ethnicity, nationality or gender are essential characteristics have in many cases been shown to have destructive or harmful results. It has been argued by some that Essentialist thinking lies at the core of many reductive, discriminatory or extremist ideologies. Psychological essentialism is also correlated with racial prejudice. In medical sciences, essentialism can lead to a reified view of identities—for example assuming that differences in hypertension in Afro-American populations are due to racial differences rather than social causes—leading to fallacious conclusions and potentially unequal treatment. Older social theories were often conceptually essentialist.