It becomes a serious problem when people begin to believe that these mental constructs reflect underlying realities. This is called essentialism. It is the belief that each of the groups we identify with our labels actually has an “essential” and immutable nature, rooted in biology or in the nature of reality. In the worst kind of case, it’s the belief that Hutus are essentially different from Tutsis, that Christian Germans are innately superior to Jews.

Essentialism can produce certain common habits of mind. Essentialists may imagine that people in one group are more alike than they really are and are more different from people in other groups than they really are. Essentialists may believe that the boundaries between groups are clear and hard and anybody adopting the culture of another group is guilty of appropriation. Essentialists may see the world divided into Manichaean dichotomies, and history as a clash of group-versus-group power struggles — clashes that demand utter group solidarity and give life meaning.

America is awash in essentialism. As the New York University philosophy professor Kwame Anthony Appiah, who writes the Ethicist column for The Times Magazine, has noted, before World War II few thought about identities the way we do today. But now it feels that contemporary politics is almost all about identity — about which type of person is going to dominate.