Historians and political scientists have argued for decades about what exactly populism is, and they haven’t always come to the same conclusions. The political theorist Isaiah Berlin warned in 1967 that “a single formula to cover all populisms everywhere will not be very helpful. The more embracing the formula, the less descriptive. The more richly descriptive the formula, the more it will exclude.” Nonetheless, Berlin identified a core populist idea: the notion that an authentic “true people” have been “damaged by an elite, whether economic, political, or racial, some kind of secret or open enemy.”

The exact nature of that enemy—”foreign or native, ethnic or social”—doesn’t matter, Berlin adds. What fuels populist politics is that concept of the people battling the elite.

The Princeton political scientist Jan-Werner Müller proposes another characteristic: “In addition to being antielitist, populists are always antipluralist,” he argues in 2016’s What Is Populism? (University of Pennsylvania Press). “Populists claim that they, and they alone, represent the people.” In that formulation, the key to understanding populism is that “the people” does not include all the people. It excludes “the enemies of the people,” who may be specified in various ways: foreigners, the press, minorities, financiers, the “1 percent,” or others seen as not being “us.”

Donald Trump casually expressed that concept while running for president, declaring: “The only important thing is the unification of the people, because the other people don’t mean anything.” During the Brexit campaign, Nigel Farage, then-leader of the United Kingdom Independence Party, predicted “a victory for real people.” Apparently, those who voted against Brexit didn’t just lose; they weren’t real people to begin with.