Philosophers have often anthropomorphized a problem by imagining a demon who accomplished a task that was difficult to understand but seemed to be possible.

Socrates' demon. Socrates introduces the first philosophical demon in the Apology. He describes at his trial how he should not be judged an unbeliever, because he hears a voice that is a god (or daimon) telling him his duty is to be a philosopher.

Nietzsche commented on this demon in Twilight of the Idols, "The Problem of Socrates," section 4:

"And let us not forget those auditory hallucinations which, as 'Socrates' demon', have been interpreted in a religious sense."

Bergson's Demon. Henri Bergson described the difference between his duration and ordinary time by invoking a demon who could speed up time.

Compton's Demon. Arthur Holly Compton imagined a daemon that controlled shutters on two photocells, one leading to a dynamite explosion, the other disabling the explosion. This anticipated Einstein's 1935 suggestion of an explosion set off by a quantum event, which led Schrödinger to invent his Cat.

Descartes' Demon. Descartes in his Meditations imagined that God might deceive us and make knowledge impossible through our senses. Rather than impute evil motives to God, he postulates a demon who does the deceiving. Our sensory evidence is thus not adequate for certain knowledge. Descartes then turns to introspection and concludes that even if the demon is deceiving his senses, he cannot be wrong about his thinking, and therefore his existence. "Cogito, ergo sum."

James' Demon. William James imagined the unbeatable chess player.

"Suppose two men before a chessboard,--the one a novice, the other an expert player of the game. The expert intends to beat. But he cannot foresee exactly what any one actual move of his adversary may be. He knows, however, all the possible moves of the latter; and he knows in advance how to meet each of them by a move of his own which leads in the direction of victory. And the victory infallibly arrives, after no matter how devious a course, in the one predestined form of check-mate to the novice's king.

Let now the novice stand for us finite free agents, and the expert for the infinite mind in which the universe lies. Suppose the latter to be thinking out his universe before he actually creates it. Suppose him to say, 'I will lead things to a certain end, but I will not now decide on all the steps thereto. At various points, ambiguous possibilities shall be left open, either of which, at a given instant, may become actual. But whichever branch of these bifurcations becomes real, I know what I shall do at the next bifurcation to keep things from drifting away from the final result I intend.'"

Frankfurt's Demon. Harry Frankfurt proposes a brain monitor and intervener to limit the alternative possibilities for action open to an agent.

Information philosophy has shown that a Frankfurt Controller cannot exist.

Landé's Demon. Alfred Landé can start new causal chains. Quantum events are causa sui.

Laplace's Demon. Pierre Simon Laplace described the great intelligence.

"Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective positions of the beings which compose it, if moreover this intelligence were vast enough to submit these data to analysis, it would embrace in the same formula both the movements of the largest bodies in the universe and those of the lightest atom; to it nothing would be uncertain, and the future as the past would be present to its eyes."

Laplace's demon must have been inspired by the extraordinary vision of Leibniz a hundred years earlier, who wrote:

"Everything proceeds mathematically...if someone could have a sufficient insight into the inner parts of things, and in addition had remembrance and intelligence enough to consider all the circumstances and take them into account, he would be a prophet and see the future in the present as in a mirror."

Information philosophy has shown that a Laplace Demon cannot exist.

Lewis' Demon. C. I. Lewis imagined a demon with infinite qualia at his disposal who deliberately manipulates our experience in an effort to deceive us. Lewis claimed that in this case "knowledge could be made difficult, but not impossible." Mill's Demon was an example. Lewis claimed that Mill confused pure analytic math with its physical application in experience.

Maxwell's Demon could defy the Second Law of Thermodynamics by sorting out fast moving atoms from slow-moving ones. With a tiny door between two containers, the demon opens the door whenever a fast moving particle approaches from the left, or a slow one from the right. Without any energy expediture (it was thought), the right container would heat up and the left cool down.

Loschmidt's Demon. An army of Maxwell demons needed to reverse instantaneously the velocities of all the particles in a gas, to show that the entropy of such a ("time-reversed") system would decrease, violating the second law of thermodynamics and vitiating Boltzmann's H-theorem.

Mendel's Demon. Contemporary writer Mark Ridley describes a randomizing demon who "stands over each gene in a parent and decides whether it will be inherited in the next generation." Where Maxwell's demon increases the order, Mendel's demon reduces it to increase the diversity needed to produce complex life forms.

Mill's Demon. John Stuart Mill's demon could, when two pairs were being added, sneak in a fifth item so that our experience would indicate that 2 + 2 = 5.
Mill hoped in this way to prove that arithmetic is not a priori.

Nietzsche's Demon. In Book Four of the Gay Science, section 341, just as he is about to introduce the Eternal Return and its great weight that adds meaning to our choices, Friedrich Nietzsche says

"What, if some day or night a demon were to steal after you into your loneliest loneliness and say to you: 'This life as you now live it and have lived it, you will have to live once more and innumerable times more; and there will be nothing new in it, but every pain and every joy and every thought and sigh and everything unutterably small or great in your life will have to return to you, all in the same succession and sequence - even this spider and this moonlight between the trees, and even this moment and I myself. The eternal hourglass of existence is turned upside down again and again, and you with it, speck of dust!'"

"Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus? Or have you once experienced a tremendous moment when you would have answered him: 'You are a god and never have I heard anything more divine.'"

Peirce's Demon. Charles Sanders Peirce imagined a tiny demon inside a jar of black and white balls who could bias the outcome of probability trials by preferentially handing up black (or white) balls. Perhaps inspired by Mill's demon.

Popper's Demon. The deaf physicist who could not only predict what Mozart and Beethoven would write, but also how their music would have been different if they had eaten lamb instead of beef...

Russell's Demon. In 1914 Bertrand Russell imagined recording instruments that could perceive the world in place of a human observer. Russell thought that his virtual observer eliminates the subjectivity of perception of what things "really" are.

"There is no theoretical limit to what can be done to make mechanical records analogous to what a person would perceive if he were similarly situated."

"It is quite unnecessary, in considering this problem, to bring in minds or sensations, the whole thing is physical."

(Russell, My Philosophical Development, 1959)

Minds are still needed. And humans record their emotions alongside their perceptions, something a machine might emulate someday. See our ERR.

Wiener's Demon. In his "Human Use of Human Beings," Cybernetics founder Norbert Wiener saw the Devil himself increasing entropy everywhere.