Essentialism—what I’ve called "the tyranny of the discontinuous mind"—stems from Plato, with his characteristically Greek geometer’s view of things. For Plato, a circle, or a right triangle, were ideal forms, definable mathematically but never realised in practice. A circle drawn in the sand was an imperfect approximation to the ideal Platonic circle hanging in some abstract space. That works for geometric shapes like circles, but essentialism has been applied to living things and Ernst Mayr blamed this for humanity’s late discovery of evolution—as late as the nineteenth century. If, like Aristotle, you treat all flesh-and-blood rabbits as imperfect approximations to an ideal Platonic rabbit, it won’t occur to you that rabbits might have evolved from a non-rabbit ancestor, and might evolve into a non-rabbit descendant. If you think, following the dictionary definition of essentialism, that the essence of rabbitness is "prior to" the existence of rabbits (whatever "prior to" might mean, and that’s a nonsense in itself) evolution is not an idea that will spring readily to your mind, and you may resist when somebody else suggests it. ...
Essentialism rears its ugly head in racial terminology. The majority of "African Americans" are of mixed race.
Sure, but the great majority are majority sub-Saharan. The minority that weren't used to prefer the Latin-style found in New Orleans where they considered themselves a middle group, but in the second half of the 20th Century, public expressions of such views became unpopular for reasons of idealistic solidarity on behalf of the black masses and/or a convenient way to prosper as the leadership of the black masses.
Yet so entrenched is our essentialist mind-set, American official forms require everyone to tick one race/ethnicity box or another: no room for intermediates.
No, actually, since the 2000 Census, the U.S. government allows people to tick as many of the racial boxes as they want. I believe there are 63 possible combinations.
On the 2010 Census, the President of the United States chose to ignore his mother's half of his family and tick only the "African-American" box.
The hilariously essentialist Census category is Ethnicity, where you are either "Hispanic" or "Non-Hispanic." Are you a Congregationalist minister and member of the Myopia Hunt Golf Club? Non-Hispanic! Are you a Tamil Brahmin? Non-Hispanic! Are you a Maori character star? Non-Hispanic! (Oh, wait, Cliff Curtis mostly plays Hispanics ... and Arabs ...)
A different but also pernicious point is that a person will be called "African American" even if only, say, one of his eight great grandparents was of African descent. As Lionel Tiger put it to me, we have here a reprehensible "contamination metaphor."
Or, these days, it's a good way to get ahead in the world, as the career of the current President shows.
But I mainly want to call attention to our society’s essentialist determination to dragoon a person into one discrete category or another. We seem ill-equipped to deal mentally with a continuous spectrum of intermediates. We are still infected with the plague of Plato’s essentialism.
Something else to keep in mind is that there is one irreducible essence in human affairs that in practice surprisingly resembles a Platonic archetype: the structure of your biological family tree. Every individual has one father and one mother, two grandfathers and two grandmothers, and so forth and so on. If you draw out the shape of the family tree of your ancestors, it is exactly the same shape as every other human's in the world. It's Platonic perfection.
The only thing messy about this is the inevitable inbreeding -- 40 generations back you have roughly a trillion slots to fill in your family tree, but there weren't a trillion people around to fill it, so some (many) of your ancestors fill multiple slots in your Platonic family tree.
Racial groups -- or partly inbred extended families -- emerge from this tension between the Platonic purity of the structure and the messy reality of the names filling the structure.