With their long, rich and varied history demons can tell us about both nature and thinking itself, says Robert P Crease
Demons might be mythical, but for hundreds of years these imaginary creatures have been fantastically useful for thinking about science and the humanities. For physicists, the most famous demon was dreamed up by James Clerk Maxwell in 1867. His creature appeared to violate the second law of thermodynamics by letting hot molecules in a box of gas go in one direction through a trap door, leaving cold molecules behind.
Inspired by the rich history of demons, my Stony Brook University colleague Elyse Graham and I created what we believe is the first university course that both studies demons and teaches students to code them. We taught the course, entitled “Demons to think with”, for the first time in spring 2025 and again in spring 2026, each time to almost 100 students. The course proved to be a great way of getting students in the humanities and sciences to talk to, work with, and learn from each other.
We surveyed demons from ancient times to the present and discussed their presence throughout art, literature and religion, where they prey on human vulnerabilities to steal souls. We also taught students about what computer engineers call “daemons”, whose malicious members prey on human vulnerabilities via clickbait and other temptations to steal passwords and social-security numbers. But what struck Graham and me most was the sheer number, variety and ambition of scientifically informed demons.
Fictional and frictional
One species was what we might call one-off demons, which explain puzzles about a specific phenomenon. Back in the early 1950s, explanations that rely on this kind of demon were parodied by the American psychologist Wendell Johnson in his 1946 book People in Quandaries: the Semantics of Personal Adjustment.
Once upon a time, Johnson wrote, people were puzzled by the fact that they could rarely find a lead pencil when they needed one – and when they did, its sharpener was sure to be filled with pencil shavings. A committee was appointed to develop a theory to explain this conundrum.
According to this made-up theory, there are lots of little people called plogglies who live underground. “At night, when people are asleep,” Johnson wrote, “the plogglies come into their houses. They scurry around and gather up all the lead pencils, and then they scamper over to the pencil sharpener and grind them all up. And then they go back into the ground.”
Even though it was impossible to know when, where and on what the plogglies will act, Johnson pointed out, this was a brilliant theory as it accounted for two mysteries at once.
Demons to think with: the rebels you mustn’t ignore
Another species of demon comprises law-governed demons. The Princeton University physicist Eric Rogers gave an example in his 1960 book Physics for the Inquiring Mind. Demons cause friction, in this explanation, because they “stand in front of things and push to stop them from moving”. These demons are transparent and too small to see, and the more demons there are, the greater the friction.
Oil reduces friction because it drowns the demons, while heavy objects crush their bones. The laws of friction spring from the fact that the demons have strict habits in the way they rush out of the pores of a surface and get crushed or drowned, and multiply and return to duty at a steady rate.
Law-governed demons play what I think of as a metaphysical role, for they explain the agency behind the regularities of a phenomenon in a way that can be divorced from the regularities themselves. Scientists can study and apply the laws of friction, for example, independently of the demons that cause it. The laws can be studied in what philosophers call the “immanent frame”, apart from whatever transcendent features are involved in putting them in that frame. That lets philosophers and scientists go their separate ways.
Stand-ins and tutors
A third category of demons are what we might call placeholder demons. In 1959 the US physicist David Pines proposed the existence of a massless, chargeless quasiparticle that is an out-of-phase collective excitation of electrons. With a nod to Maxwell, he named it Pines’ demon, reinforcing the term with an improvised acronym (Distinct Electron Motion + on).
Back in 2023 a team of researchers at the University of Illinois Urbana-Champaign discovered the particle – a kind of plasmon – using specialized electron spectroscopy in strontium ruthenate. Pines’ creature was a placeholder demon because it simply gave a name for something that we know is lurking in the immanent frame and only have to find it.
The emergence of quantum physics at the start of the 20th century spawned a new generation of instructional demons
Physicists, finally, use instructional demons. Its most famous member is Maxwell’s demon, mentioned at the beginning of this article. Even a century and a half later it still teaches theorists about the intersection of energy, entropy and information. But the emergence of quantum physics at the start of the 20th century spawned a new generation of instructional demons, including two from Nobel-prize-winning physicists.
In 1911 Marie Curie wondered about a demon that could manipulate quantized packets of energy, while in 1935 Arthur Compton imagined a demon that could open a door for “good” photons and slam the door on “bad” photons that might trigger a stick of dynamite. Working out how far these demons could get – whether law-abiding, law-breaking or law-bending – teaches physicists much about the intersection of information, entropy and quantum mechanics. Instructional demons teach theorists not so much about pieces of the immanent frame as about the structure of the frame itself.
The critical point
As the science historian Jimena Canales showed in her 2020 book Bedeviled: A Shadow History of Demons in Science, demons inhabit even the most methodical and evidence-based thinking. Scientific demons are like other kinds of demons in that they are wily supernatural creatures with superhuman powers. They are unlike other kinds of demons, however, in that they don’t try to take advantage of human vulnerabilities to change us but try to take advantage of nature’s vulnerabilities to change it.
Postulating demons with supernatural powers is a way that scientists have used to explore nature. Studying scientific demons therefore not only tells us about nature but scientific thinking itself.
- If you enjoyed this article, give our demon cryptic crossword a go.
