Laplace's Demon

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For an all-seeing, all-knowing being, the future is as clear as the past

Perhaps you, like me, have once considered a thought experiment:

Given all the laws of nature and given the current state of everything in the universe, in principle, you could figure out exactly what was going to happen. The laws of nature would just keep things moving as expected, and events in the universe would unfold predictably and imperturbably. What about free will and all that?

This idea of a being that knew all the laws and the current states of everything, and therefore how everything would play out, is called Laplace’s Demon. It was named after an extremely smart French mathematician and polymath, Pierre-Simon Laplace, who once wrote (translations vary a little):

“We ought then to regard the present state of the universe as the effect of its anterior state and as the cause of the one which is to follow.

Given for one instant an intelligence which could comprehend all the forces by which nature is animated and the respective situation of the beings who compose it—an intelligence sufficiently vast to submit these data to analysis—it would embrace in the same formula the movements of the greatest bodies of the universe and those of the lightest atom;

for it, nothing would be uncertain and the future, as the past, would be present to its eyes.”

— Pierre-Simon Laplace, A Philosophical Essay on Probabilities

Determinism is the idea that everything is pre-determined and nothing is left to chance. Laplace’s Demon thought experiment represents the extreme side of that.

There are a few challenges to this idea since then, including the practical unpredictability of chaotic systems, quantum mechanics, the difficulty of measuring without disturbing, as well as philosophical and religious ideas about free will. Even if the future did follow perfectly from the past, in practice we couldn’t work it out.

Notes on Laplace’s Demon

I learned about Laplace’s Demon in David Spiegelhalter’s The Art of Uncertainty after he joined us on the podcast about microlives .

Laplace’s Demon is less well-known than Maxwell’s demon — a teeny being that appears to violate the second law of thermodynamics.

More generally, Laplace’s Demon is a thought experiment—a hypothetical scenario that helps stress-test and illuminate ideas.

Related Ideas to Laplace’s Demon

Also see:

You can read a translation of Laplace’s words in his article A Philosophical Essay on Probabilities on Project Gutenberg (this appears near the beginning).

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The famous moral dilemma: switch the tracks or not? (Here's a combined version of the Trolley Problem and 3 Variants) Imagine you’re the driver of a runaway trolley whose brakes have failed. Ahead on the track, five people are working and will be killed if you do nothing. But you notice a side-track—if you switch the trolley to it, you’ll avoid the five, but one person on that track would be killed instead. Should you pull the lever and switch the tracks—killing one to save five? This is the start of the famous moral dilemma of The Trolley Problem. The fictitious scenario, and others like it, are so effective because they force us to confront our internal moral compass: what seems like the right thing to do, and why? Trolley Problem Variants As we formulate a position on a scenario, it's possible to devise variant scenarios that test our moral reasoning more sharply. For example, in the original problem, many people might suggest that it's OK for the driver to choose to save five people's lives at the expense of the one person on the other track on the basis that killing five people is worse than killing one. Where would you stand on these other variants? The Bystander at the Switch Sketch of the Bystander at the Switch Variant You’re no longer the driver. This time, you’re simply passing by when you see a runaway trolley heading toward five people. The trolley is driverless, and you’re standing next to a switch that could divert the train to another track—where one person would be killed instead. Is it morally acceptable to intervene? Does it feel different when you’re not the driver but an onlooker? The Heavy Man Problem Sketch of the Heavy Man Variant Or suppose that you were on a bridge above the track when you saw the train on its way to crash through the five people working on the track. There’s no switch—but next to you stands a very heavy man. If you push him off the bridge, his body would stop the trolley and save the five. Most people react more strongly against this version, even though the outcome is similar: one life traded for five. (This variant was originally the 'Fat Man' problem, but it's not important that the person is fat, just that they're heavy enough to stop the trolley). The Mafia Problem Sketch of the Mafia Variant Suppose once again you were the bystander at the switch passing the track in this deadly scene. This time, however, you see that the five people on the track are not workmen but hardened criminals. What's more, they've tied an innocent person to the other track. If you switch the trolley, the innocent person dies. If you do nothing, the five criminals die. Does who the people are affect your decision? How to Decide? It may be clear to you what the right path to choose is. And yet, your position may differ from someone else faced with the same scenario. The philosopher Philippa Foot introduced the Trolley Problem in a 1967 paper discussing abortion (PDF), along with several options for reasoning about it. Judith Jarvis Thomson invents some of the further scenarios (such as the mafia case) and alternative arguments in a 1985 paper, The Trolley Problem (pdf). Some possible positions and distinctions you might keep in mind include: Passive vs Active Harm Is it morally different to let five people die (by doing nothing) than to actively cause one person’s death (by switching tracks or pushing someone)? Are you killing or are you failing to save? Is there a difference in what we do and what we allow? Negative Duties vs Positive Duties Some argue there’s a moral difference between duties not to harm (negative duties) and duties to help (positive duties). Avoiding Injury vs Bringing Aid Is there a difference when you are making choices that save different numbers of people rather than making choices where people will die as a direct result of your choices? For example, suppose you face a choice of rescuing a large group of people while leaving another small group to die. Or what if a villain asked you to sacrifice an innocent person, or they will kill five others. Rights vs Utility Does your action violate someone’s rights? Even if the outcome is better overall, is it acceptable to use a person as a means to an end? Is it a matter of degrees? For example, violating an individual's rights by pushing them off a bridge versus stealing from them as a route to save someone. Doctrine of Double Effect Philippa Foot discusses the Doctrine of Double Effect—that our actions may have intended consequences and other outcomes that are foreseeable but not intended—the distinction between "direct" and "oblique intention". In the case of the runaway trolley, it's "one thing to steer towards someone foreseeing that you will kill him and another to aim at his death as part of your plan." Or, for example, in surgery to save a mother during childbirth, the death of a child may be foreseen but not intended. Dilemmas While fictitious moral dilemmas are intriguing thought experiments, real dilemmas surround us. A general orders his troops on a dangerous mission. Adjusting the speed limit on a dangerous road. People die from driving too fast, but walking isn't an option—what's the appropriate risk of death we're comfortable with? Who should receive aid? Who should be rescued first? Should a pilot steer a plane that's about to crash into a less populated area? I remember, in the film Beneath Hill 60 (spoiler), a commander must decide whether to detonate underground munitions to execute the battle plan, knowing that one of his team members is trapped. I find moral dilemmas to be at once fascinating, puzzling, troubling, and uncomfortable. Related Ideas to the Trolley Problem The Front Page Test Equality and Equity The Prisoner's Dilemma Learn More About the Trolley Problem You can, and should, have fun with absurd trolley problems at the brilliant neal.fun—and also see what other choices people made In Michael Sandel's course and book, Justice, he discusses the trolley problem and many other difficult questions of ethics and morals and how to reason about them Both Philippa's and Judith's original papers are quite readable: Philippa Foot, The Problem of Abortion and the Doctrine of the Double Effect (PDF) Judith Jarvis Thomson, The Trolley Problem (PDF) Prints of the Trolley Problem Trolley Problem standalone prints Trolley Problem and Variants printsThe famous moral dilemma: switch the tracks or not? (Here's a combined version of the Trolley Problem and 3 Variants) Imagine you’re the driver of a runaway trolley whose brakes have failed. Ahead on the track, five people are working and will be killed if you do nothing. But you notice a side-track—if you switch the trolley to it, you’ll avoid the five, but one person on that track would be killed instead. Should you pull the lever and switch the tracks—killing one to save five? This is the start of the famous moral dilemma of The Trolley Problem. The fictitious scenario, and others like it, are so effective because they force us to confront our internal moral compass: what seems like the right thing to do, and why? Trolley Problem Variants As we formulate a position on a scenario, it's possible to devise variant scenarios that test our moral reasoning more sharply. For example, in the original problem, many people might suggest that it's OK for the driver to choose to save five people's lives at the expense of the one person on the other track on the basis that killing five people is worse than killing one. Where would you stand on these other variants? The Bystander at the Switch Sketch of the Bystander at the Switch Variant You’re no longer the driver. This time, you’re simply passing by when you see a runaway trolley heading toward five people. The trolley is driverless, and you’re standing next to a switch that could divert the train to another track—where one person would be killed instead. Is it morally acceptable to intervene? Does it feel different when you’re not the driver but an onlooker? The Heavy Man Problem Sketch of the Heavy Man Variant Or suppose that you were on a bridge above the track when you saw the train on its way to crash through the five people working on the track. There’s no switch—but next to you stands a very heavy man. If you push him off the bridge, his body would stop the trolley and save the five. Most people react more strongly against this version, even though the outcome is similar: one life traded for five. (This variant was originally the 'Fat Man' problem, but it's not important that the person is fat, just that they're heavy enough to stop the trolley). The Mafia Problem Sketch of the Mafia Variant Suppose once again you were the bystander at the switch passing the track in this deadly scene. This time, however, you see that the five people on the track are not workmen but hardened criminals. What's more, they've tied an innocent person to the other track. If you switch the trolley, the innocent person dies. If you do nothing, the five criminals die. Does who the people are affect your decision? How to Decide? It may be clear to you what the right path to choose is. And yet, your position may differ from someone else faced with the same scenario. The philosopher Philippa Foot introduced the Trolley Problem in a 1967 paper discussing abortion (PDF), along with several options for reasoning about it. Judith Jarvis Thomson invents some of the further scenarios (such as the mafia case) and alternative arguments in a 1985 paper, The Trolley Problem (pdf). Some possible positions and distinctions you might keep in mind include: Passive vs Active Harm Is it morally different to let five people die (by doing nothing) than to actively cause one person’s death (by switching tracks or pushing someone)? Are you killing or are you failing to save? Is there a difference in what we do and what we allow? Negative Duties vs Positive Duties Some argue there’s a moral difference between duties not to harm (negative duties) and duties to help (positive duties). Avoiding Injury vs Bringing Aid Is there a difference when you are making choices that save different numbers of people rather than making choices where people will die as a direct result of your choices? For example, suppose you face a choice of rescuing a large group of people while leaving another small group to die. Or what if a villain asked you to sacrifice an innocent person, or they will kill five others. Rights vs Utility Does your action violate someone’s rights? Even if the outcome is better overall, is it acceptable to use a person as a means to an end? Is it a matter of degrees? For example, violating an individual's rights by pushing them off a bridge versus stealing from them as a route to save someone. Doctrine of Double Effect Philippa Foot discusses the Doctrine of Double Effect—that our actions may have intended consequences and other outcomes that are foreseeable but not intended—the distinction between "direct" and "oblique intention". In the case of the runaway trolley, it's "one thing to steer towards someone foreseeing that you will kill him and another to aim at his death as part of your plan." Or, for example, in surgery to save a mother during childbirth, the death of a child may be foreseen but not intended. Dilemmas While fictitious moral dilemmas are intriguing thought experiments, real dilemmas surround us. A general orders his troops on a dangerous mission. Adjusting the speed limit on a dangerous road. People die from driving too fast, but walking isn't an option—what's the appropriate risk of death we're comfortable with? Who should receive aid? Who should be rescued first? Should a pilot steer a plane that's about to crash into a less populated area? I remember, in the film Beneath Hill 60 (spoiler), a commander must decide whether to detonate underground munitions to execute the battle plan, knowing that one of his team members is trapped. I find moral dilemmas to be at once fascinating, puzzling, troubling, and uncomfortable. Related Ideas to the Trolley Problem The Front Page Test Equality and Equity The Prisoner's Dilemma Learn More About the Trolley Problem You can, and should, have fun with absurd trolley problems at the brilliant neal.fun—and also see what other choices people made In Michael Sandel's course and book, Justice, he discusses the trolley problem and many other difficult questions of ethics and morals and how to reason about them Both Philippa's and Judith's original papers are quite readable: Philippa Foot, The Problem of Abortion and the Doctrine of the Double Effect (PDF) Judith Jarvis Thomson, The Trolley Problem (PDF) Prints of the Trolley Problem Trolley Problem standalone prints Trolley Problem and Variants prints

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