Could a perfect mind predict everything?
Imagine an intellect that knows the position and motion of every particle. In a deterministic universe, nothing would be uncertain to it, and the future would be as plain as the past.
- Reading time
- 4 min
- Difficulty
- Intermediate
- Branch
- Free will
- First posed
- Pierre-Simon Laplace, A Philosophical Essay on Probabilities, 1814
The setup
In 1814 the French mathematician Pierre-Simon Laplace described an imaginary intelligence 'which could comprehend all the forces by which nature is animated and the respective situation of the beings who compose it'. If this intellect were vast enough to analyse all that data, Laplace wrote, 'nothing would be uncertain and the future just like the past would be present before its eyes.'
Later writers called this being Laplace's demon. It is the vivid embodiment of determinism: if the universe runs on fixed laws, then the present contains the entire future, and a sufficiently powerful mind could read it off.
The demon would know what you will have for dinner on your eightieth birthday, the words of a poem not yet written, and the outcome of every election. Your choices would be, for it, as predictable as the orbit of the moon.
“For such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.”
Pierre-Simon Laplace, A Philosophical Essay on Probabilities (1814)
Why it matters
Laplace's demon makes the stakes of determinism concrete. Many people find it harder to believe they are free when they imagine a being who already knows what they will do.
It also helps separate two ideas that are easily confused: determinism (the future is fixed by the past and the laws) and predictability (someone could know the future in advance). As physics and logic have shown, these come apart.
Key terms
- Laplace's demonAn imagined intellect that knows every particle's position and every force at one moment, and so could calculate the entire past and future. A symbol of determinism.
- DeterminismThe thesis that the state of the world at any moment, together with the laws of nature, fixes everything that happens afterwards.
- Chaos (deterministic)Behaviour of a fully deterministic system that is so sensitive to its starting conditions that long-term prediction is practically impossible.
- IndeterminismThe thesis that not every event is fixed by prior events and the laws of nature; some involve genuine chance.
- Self-referenceA sentence, set or system referring to itself. Many paradoxes, such as the liar and Russell's, arise from it.
The positions
In principle, yes
If the universe is deterministic, a sufficiently powerful intellect outside the universe could in principle predict everything.
Physics forbids it
Quantum indeterminacy, chaos and the limits of measurement make Laplace's demon impossible even in principle.
Logic forbids it
No predictor that is part of the universe can predict everything, even if the universe is deterministic, because predictions about itself and its effects lead to contradiction.
Arguments and objections
In principle, yes
- Best argument
- Determinism just means the state of the world at one time, plus the laws, fixes every later state. A being with complete knowledge and unlimited computational power, standing outside the system, would face no barrier in principle. Practical difficulties, however vast, do not affect the conceptual point.
- Strongest objection
- Modern physics is not straightforwardly deterministic. In the standard interpretation of quantum mechanics, some events are irreducibly probabilistic, and even complete knowledge of the present would yield only probabilities for the future.
- The reply
- Determinists reply that some interpretations of quantum mechanics, like Bohmian mechanics or many worlds, are deterministic, so the question is not settled by physics alone.
Physics forbids it
- Best argument
- The uncertainty principle limits how precisely position and momentum can be simultaneously defined. Chaotic systems amplify tiny uncertainties exponentially, so the demon would need infinite precision. And in standard quantum mechanics, some outcomes are not determined by anything.
- Strongest objection
- Chaos alone does not defeat a demon with truly perfect information; it only defeats finite observers. The real obstacle is quantum indeterminacy, which depends on the interpretation of quantum mechanics.
Logic forbids it
- Best argument
- Suppose the demon predicts what you will do and tells you. You can do the opposite. So the demon cannot publish correct predictions about anyone who hears them. Even without telling anyone, a predictor inside the universe would have to model itself modelling itself, an infinite regress. David Wolpert proved in 2008 that no inference device within a universe can correctly predict everything about that universe.
- Strongest objection
- This shows that an internal predictor fails, not that the future is open. A demon outside the universe, which does not interact with it, escapes the argument.
Who argued what
- In principle, yes Pierre-Simon Laplace, Baruch Spinoza
- Physics forbids it Werner Heisenberg, Ilya Prigogine
- Logic forbids it David Wolpert, Karl Popper
Variations
The demon tells you
The demon hands you a sealed envelope containing a prediction of what you will do in the next minute: raise your left hand or your right. You read it. Whatever it says, you do the other. Either the demon cannot predict you, or it cannot tell you. What does this show about determinism?
God's foreknowledge
Medieval philosophers faced a theological version. If God knows everything you will do, how can you do otherwise? Boethius answered that God is outside time and sees all moments at once, as an observer on a mountaintop sees the whole road, so God's knowledge does not cause your choices any more than seeing someone walk causes their walking.
Go deeper
Laplace's own point
Laplace introduced the demon in an essay about probability. His point was that probability measures our ignorance, not chance in nature. Because we cannot know all the causes, we use probability to reason about outcomes. The demon would have no need of probability at all.
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: Causal Determinism
- WikipediaLaplace's demon
- BookA Philosophical Essay on Probabilities, Pierre-Simon Laplace
- BookChaos: Making a New Science, James Gleick
Open questions
- If something could predict your every choice, would that make you less free?
- Is a future that no one can know any more open than one that someone could?
- Would you want to read the demon's prediction of your life?