Irrationality: A History of the Dark Side of Reason by Justin E.H. Smith Princeton University Press, 330 pp., $29.95

How enlightened was the Enlightenment? Not a few critics have seen it as profoundly benighted. For some, it was a seedbed for modern racism and imperialism; the light in the Enlightenment, one recent scholar has suggested, essentially meant “white.” Voltaire emphatically believed in the inherent inferiority of les Nègres, who belonged to a separate species, or at least breed, from Europeans—as different from Europeans, he said, as spaniels from greyhounds. Kant remarked, of something a Negro carpenter opined, that “the fact that he was black from head to toe was proof that what he said was stupid.” And David Hume wrote, in a notorious footnote, that he was “apt to suspect” that nonwhites were “naturally inferior to the whites,” devoid of arts and science and “ingenious manufactures.”

The more general critiques take up larger intellectual currents in the eighteenth century. The era’s systematic forays into physical anthropology and human classification laid the foundation for the noxious race science that emerged in the nineteenth century. So did the rise of materialism: it became harder to argue that our varying physical carapaces housed equivalent souls implanted by God. A heedless sense of universalism, in turn, might encourage the thought that the more advanced civilizations were merely lifting up those more backward when they conquered and colonized them.

For critics like John Gray, the Enlightenment’s self-satisfied rationalism and belief in progress were bound to fuel notions of racial and civilizational hierarchy, and can be linked to twentieth-century totalitarianism. And its toll continues: liberal rationalism is, in Gray’s view, an impoverished creed that has asphyxiated richer forms of life. That’s why, he says, Western societies that rely on liberal Enlightenment values “are plagued with anomie and nihilism.”

Yet these objections don’t settle the matter. The expressions of prejudice from Kant and Hume are worth noting not because they’re peculiar to the so-called Enlightenment project but precisely because they aren’t: so commonplace were such views that even these rarefied intellects weren’t immune to them. What distinguished Voltaire from other Europeans was not his assumptions about the superiority of their stock but his eloquent opposition to slavery and colonialism. The grand projet of the era, the Encyclopédie, was rude about Africans but also filled with abolitionist fervor. Hume was no fan of slavery, mercantilism, or the apparatus of imperialism; Adam Smith, his compatriot, was more outspoken about these evils still.

As for Kant—well, his views seem to have evolved. In her study Kant and Cosmopolitanism (2012), the Dutch scholar Pauline Kleingeld persuasively argued that, in the 1790s, he moved away from hierarchical notions of human difference, perhaps partly under the influence of interlocutors like Georg Forster and Johann Gottfried Herder. He now offered a clear rebuke to slavery, imperial conquest, and great-power dominion. Given that slavery and imperialism are age-old practices, it may be more…