It is difficult to count demons. In the Gospel of Mark, when Jesus meets a man on the far side of the Sea of Galilee who is possessed, he asks the demon to identify itself. It replies: “My name is Legion, for we are many.” But how many? The thirteenth-century German abbot Richalmus suspected the number of demons was incalculable, as numerous as grains of sand in the sea. Three centuries later, when the Dutch physician Johann Weyer composed his demonology, he identified some sixty-nine demons by name, who commanded millions of others: at least eleven hundred and eleven distinct legions, each with six thousand six hundred and sixty-six demons. Around the same time, the German theologian Martin Borrhaus reached a very different estimate: two trillion six hundred and sixty-five billion eight hundred and sixty-six million seven hundred and forty-six thousand six hundred and forty-four demons.

Others scholars avoided a head count, choosing instead to organize demons into typologies and hierarchies, as Dante Alighieri did in the Inferno and King James did in his “Daemonologie,” published nearly a decade before he commissioned a new translation of the Bible. According to such taxonomies, demons were a busy bunch, tasked with everything from promoting quarrels, discord, and war (the work of a demon called Bufas) to inserting errors into the manuscripts of scribes and keeping tabs on the mispronunciations of preachers during worship (the work of Titivillus).

Both quantitative and qualitative demonologies have largely fallen out of favor these days, but the historian of science Jimena Canales has just published one. “Bedeviled: A Shadow History of Demons in Science” (Princeton University Press) is not a survey of Baal, Stolas, Volac, and their kin. Instead, Canales has gathered together in one book demons with very different origins and responsibilities—among them the scientist James Clerk Maxwell’s demon, the physicist David Bohm’s demon, the philosopher John Searle’s demon, and the naturalist Charles Darwin’s demon. These demons came into being at some of the world’s leading universities and were promulgated in the pages of Science and Nature. They are not supernatural creatures; rather, they are particular kinds of thought experiments, placeholders of sorts for laws or theories or concepts not yet understood. Like the demon Jesus met, though, these are legion; at the very same time that science was said to be demystifying the world, Canales shows us, scientists were populating it all over again with the demonic.