Science’s Demons, from Descartes to Darwin and Beyond

How supernatural conceptions have advanced our understanding of the natural universe.

By Casey CepJanuary 8, 2021 New Yorker Magazine

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.

According to Canales, a faculty member at the University of Illinois at Urbana-Champaign, modern demonology began with René Descartes, who imagined a demon into being in his “Meditations on First Philosophy,” from 1641. The French philosopher was positing a thought experiment most often described today as the brain in a vat: however, instead of wondering if he was just a disembodied brain experiencing a simulated reality, Descartes proposed that “some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me.” Said demon could alter our senses and convince us of falsehoods, so that what we see, hear, or feel might not be real. Because anything might be a deception, we must assume everything is, and only through extreme skepticism can we distinguish the real from the unreal.

Descartes’s demon was not immediately followed by others, but, in 1773, the French mathematician Pierre-Simon Laplace proposed a thought experiment of his own. He imagined a mysterious entity “who, for a given instant, embraces all the relationships of the beings of this universe.” With that single instant of complete knowledge, Laplace wrote in an article on calculus, this entity “could determine for any time taken in the past or in the future the respective position, the movements, and generally the attachments of all these beings.” Because Laplace’s demon knew the present location of every single thing in the universe and all the forces acting on them, it could infer everything that had already happened and everything that would happen in the future.

Several decades before, John Locke had posited that, other than God, only angels and spirits might have such total knowledge. But Laplace argued that the universe was stable and predictable—this was why Edmond Halley could determine the regular arrival of a comet—and that, as a result, mathematical analysis could help us understand the universe in its entirety. It was therefore perfectly reasonable, even for those of us who don’t possess infinite information and limitless cognitive power, to use what information we do have and what cognition we can summon to make sense of the world. Laplace’s faith in scientific determinism helped inspire the creation of machines that could do the kinds of computations he attributed to his demon. Charles Babbage read Laplace’s work, and cited it in accounts of his “Difference Engine” and “Analytical Engine,” machines designed to perform calculations; Babbage’s friend Ada Lovelace, who was tutored by Laplace’s English translator, grasped the implications of Babbage’s engines, and encouraged him to find additional applications for what are now considered some of the earliest computers.

Darwin knew Babbage, too, and talk of demons and determinism might well have helped shape his account of evolution. Darwin’s notes on the subject originally included “a being infinitely more sagacious than man,” one “with penetration sufficient to perceive differences in the outer and innermost organization quite imperceptible to man, and with forethought extending over future centuries to watch with unerring care and select for any object the offspring of an organism produced under the foregoing circumstances.” This strange being disappears entirely from the final version of “On the Origin of Species,” where only the theory of natural selection appears, absent any miraculous causes or supernatural forces.

Darwin eventually had a demon named for him, anyway: a hypothetical organism with infinite reproductive capacity and longevity, existing with no biological constraints, useful as a thought experiment for understanding evolutionary theory. By then, demons had proliferated across a wide range of scientific fields. Canales argues that this is partly because of the popularity of one of them in particular: the demon devised by the British physicist James Clerk Maxwell. The first version of this creature, described in a letter to a colleague in 1867, is only “a very observant and neat-fingered being,” not yet a demon. That being stood between two containers, opening and closing a door between them, allowing only certain molecules to pass, sorting the fast ones from the slow ones without exerting any energy, and thereby making one container warmer than the other. Maxwell had imagined what others called a perpetual-motion machine, one capable of reversing entropy.

Later, on a page of notes labelled “Concerning Demons,” Maxwell clarified how his “being” became a demon:

1. Who gave them this name? Thomson.

2. What were they by nature? Very small but lively beings incapable of doing work but able to open and shut valves which move without friction or inertia.

3. What was their chief end? To show that the 2nd law of Thermodynamics has only a statistical certainty.

There was a fourth item in Maxwell’s list, addressing the question of whether the demon’s only occupation was changing temperature. To this, he responded that the demon could also change pressure, but such a task required less intelligence; it could be performed by “a valve like that of the hydraulic ram.” That hypothetical valve might never have moved the imagination of physicists, much less biologists, computer scientists, economists, and sociologists, but the demon (so named, as Maxwell tells us, by William Thomson, better known as Lord Kelvin) certainly did.