You can dance around the question of whether Donald Trump’s “he’s a Mexican” criticism of federal judge Gonzalo Curiel, born in Indiana, is technically “racist.” There is no such race as Mexican, after all, and the citizenry of Mexico ranges from blue-eyed blonds to those with dark skin and jet-black hair.

But what’s clear about Trump’s argument, and what is deeply un-American about it, is its essentialism. Trump is saying that because of who you are, in an ethnic or hereditary sense, he will make judgments about what you think and what role in society you can play. The related assumption, as Garrett Epps explains to irrebuttable effect, is that people’s public roles cannot be separated from their ethnic or religious identities. A “Mexican” judge will think and act as a Mexican, not as a judge.

There is no more un-American concept.

For citizens of the United States, essentialism should mean: Who you are, is American. And everything about your place in this society should depend on what you do and say, how you contribute and to what you aspire, rather than to whom you were born. This is the revolutionary American idea. It is the most truly exceptional part of American exceptionalism. And the long struggle of American history has been to match reality to that ideal. This is the concept of becoming a more perfect union laid out in the Constitution, which ranges from extirpating the original sin of slavery, to dealing with slavery’s ongoing effects, to the continuing effort to open opportunities, under fair conditions, for Americans regardless of background.