QuestionsAre we free?The intellect that would see the whole future
Are we free?No free will

The intellect that would see the whole future

Laplace's demon

Pierre-Simon de Laplace 1749 · 1827An intellect knowing all forces and positions would see the future like the past.
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What it claims

Free will in the strong sense does not exist: believing that one determined oneself "of itself and without motives" is an illusion of the mind. And for a vast enough intellect, our future would be as readable as our past.

Why

Every effect has a cause, even the will: the present follows from the past, so an intelligence knowing everything could see the future.

The same idea through history
Benedictus de Spinoza1677In the Ethics (I, prop. 29), he writes that in nature there is nothing contingent: everything is determined to exist and act in a definite way. The will is no exception.
Gottfried Wilhelm Leibniz1710He states the principle of sufficient reason: nothing happens without a reason explaining why it is so rather than otherwise. Laplace cites him in support of his argument.
In short

"The intellect that would see the whole future" is an argument by Pierre-Simon de Laplace (A Philosophical Essay on Probabilities, 1814, chapter "Concerning probability") on the question "Are we free?" (answer: no free will). Its claim: Free will in the strong sense does not exist: believing that one determined oneself "of itself and without motives" is an illusion of the mind. And for a vast enough intellect, our future would be as readable as our past. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P5). AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

A Philosophical Essay on Probabilities, 1814, chapter "Concerning probability"
Valid with an implicit premise

Laplace was an astronomer and mathematician, not a professional philosopher. Yet in 1814 he wrote a page that became famous. If every state of the world is the effect of the previous one, then whoever knew everything at one given instant could calculate all the rest: the future as well as the past. Laplace does not call this intellect a "demon" (the word came later). Nor does he say that we are this intellect: he says science comes close to it without ever reaching it. But he draws a consequence for freedom: even the freest will cannot act without a motive that determines it.

P1Nothing begins to be without a cause that produces it. This rule, the principle of sufficient reason, holds even for actions we consider indifferent.
P2The will is not set apart: even the freest cannot act without a determining motive. A choice with no motive at all would be an effect without a cause.
P3So the present state of the universe, wills included, is the effect of its previous state and the cause of the one that follows.
P4An intellect that knew at one instant all the forces of nature and the position of all beings, and was vast enough to calculate, would see the future like the past.
P5 · implicit premiseA choice that is the effect of the previous state of the world is not a choice that nothing determines: it is not free will in the strong sense.
Therefore free will in the strong sense does not exist: believing that one determined oneself "of itself and without motives" is an illusion of the mind. And for a vast enough intellect, our future would be as readable as our past.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P5, the computer finds a world where everything is determined and predictable, and yet there is free will: the conclusion no longer follows.
With P5 the conclusion follows. P1 to P3 give determinism; P4 gives prediction; P5 links determinism to free will. Without P5, a compatibilist could accept everything else and say: a determined choice can still be free. That is where the debate lies, along with whether P1 is true of the real world (quantum physics).
Analogy. Think of a film that is already shot. For the characters, the story seems to open up scene by scene. But someone holding the whole reel already knows the end, and the beginning. Laplace's intellect is the viewer who sees the whole reel at once.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

H
Heisenberg (quantum mechanics) 1927 · attacks premise 1

The laws of quantum mechanics give only probabilities. According to Heisenberg, the position and velocity of a particle cannot both be known with perfect precision, and he concludes that the law of causality no longer holds at the atomic scale. If the world is not strictly determined, Laplace's intellect cannot exist.

Laplace replies
Hume 1748 · A reply already given by Hume, long before quantum physics
If liberty is opposed to necessity, not to constraint, it is the same thing as chance, which is universally allowed to have no existence. An event due to pure chance is not a free choice.
Galen Strawson 1994
It is absurd to suppose that indeterministic or random factors, for which one is by hypothesis in no way responsible, can in themselves contribute in any way to one's being truly morally responsible for how one is.
The debate goes on (our summary) The debate stays open on two points. In physics, the interpretation of quantum mechanics is disputed: some versions (such as de Broglie and Bohm's) remain deterministic. In philosophy, "libertarian" authors argue that a little indeterminism, in the right place in the brain, could make free will possible. What is certain: the quantum objection removes P1, but does not by itself show that we are free.
L
Lorenz (chaos theory) 1963 · attacks premise 4

Even in a perfectly deterministic system, two almost identical starting states can lead to very different states. One would need to know the initial state with infinite precision to foresee the distant future, which no real being can do.

Laplace replies
Laplace 1814 · Laplace had already foreseen the limit
The human mind offers only a feeble sketch of this intellect, from which it will always remain infinitely removed. Laplace's intellect is thus not an instrument we possess, but the idea of a mind without limits.
The debate goes on (our summary) Predicting is one thing, being determined is another. Lorenz's equations are deterministic: it is in the title of his paper. Chaos shows that we can never predict, not that the future is open. One point remains to discuss: is a demon who knew the state of the world with infinite precision possible, even in principle? Laplace's argument assumes it is.
Hume (compatibilism) 1748 · attacks premise 5

Being free does not require that nothing determine our choice. By liberty we can only mean a power of acting or not acting according to the determinations of the will. That power can exist in a world where everything is caused.

no reply
We have not yet found a direct reply by Laplace in the texts.
The debate goes on (our summary) Laplace speaks of free will in the strong sense, that of a will that would determine itself "of itself and without motives". Hume answers in advance that this is not the only possible freedom. The two authors are thus not talking about the same thing: everything depends on what "free" means.
What today's voices say
Defend it
Sabine HossenfelderShe says quantum indeterminism does not rescue free will: random processes are not influenced by anything, so they are not influenced by your free will either.
Criticize it
Daniel DennettHe presents Laplace's determinism, in which the next instant can be predicted to a certainty, but says determinism is 'simply irrelevant' to what matters for free will: what is true across the neighbouring possible worlds.