This is true, but this approach also has a flaw: it assumes people’s thoughts and behaviors are consistent and coherent enough for rigorous analysis. Smith’s ethnic “essences” are those of a philosopher — they’re based on logical definitions (this is human, that is not) and, being eternal essences, they’re forever. “Subhumanity,” he writes, “is typically thought to be a permanent condition. Subhumans can’t become humans any more than frogs can become princes.”

These requirements leave much of the reality of dehumanization outside Smith’s tent. For example, he writes of the immense guilt of soldiers who have dehumanized their enemy in combat, but then, back in peacetime, recognize their victims as actual people, just like the folks at home. Shrewdly, he notes how military training and rituals aim to undermine this feeling that the enemy is human. But he seems not to recognize that this sort of soldier — who sees his enemy as human, then subhuman, then human again — cannot exist if dehumanization depends on a stable belief in permanent essences.

Similarly, in Smith’s account of slavery, the ancient Romans viewed their slaves as nonhuman frogs that could never turn into human princes. But for centuries, Romans were accustomed to former slaves re-entering society and rising to responsible and even powerful positions.

Then too, because essences must be consistent and well defined, Smith is forced to claim that all dehumanization makes its targets seem, as his title says, “less than human.” But many forms of dehumanization work by making their targets out to be more than human — to be ageless vampires, for instance, or world-spanning conspirators who control the United Nations and the Federal Reserve.

All these troubles arise because Smith’s insistence on immutable essences is only half right: people do categorize themselves and others using essences, but there’s nothing immutable about them. If, like trained philosophers, we could settle for good who is essentially human and who is a zombie vampire squid, we wouldn’t have, or need, this drama of dehumanization, rehumanization, then more dehumanization, and so on. Instead, the who-is-and-isn’t-human question is never truly settled. In fact, it is the dynamic, even mercurial nature of “real human” status that makes this mystery of our psychology so fascinating. What Amenemhet wants us to remember isn’t that he thought Asiatics were dogs, but that he made them act like dogs.

“We don’t humiliate vermin,” Adam Gopnik has observed, “or put them through show trials, or make them watch their fellow-vermin die first.” Right: we do that to people. Because we readily see others as human, we need reminding that our enemies are supposedly different. Which often works, because we also readily see others as not human. Smith has explored the nature of those conceptual boxes “human” and “not human.” But what really needs explaining is the constant, restless travel that the mind makes between them.