I had no need of that hypothesis

The God hypothesis, unnecessary in astronomy; Laplace's reply to Napoleon

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

In astronomy, the hypothesis of a divine design is not necessary. Where we do not yet know, it is wiser to admit our ignorance than to fill it with imagined causes.

Why

The solar system is stable by its own laws and its arrangement may follow from them: better admit ignorance than posit God.

The same idea through history

Isaac Newton1713Newton concludes from the arrangement of the planets that it is the work of an intelligent and all-powerful being. Laplace quotes this passage in order to answer it: it is the starting point of his argument.

Gottfried Wilhelm Leibniz1715According to Laplace, Leibniz held that calling on God to put the solar system back in order means "having very narrow ideas of the wisdom and power of God" (the debate took place in his letters to Samuel Clarke, Newton's spokesman, in 1715-1716; the phrase is given by Laplace, not checked in Leibniz). Laplace also quotes Newton's retort against Leibniz's pre-established harmony, and dismisses both: "posterity has not accepted these idle hypotheses".

Pierre-Simon de Laplace1835Laplace states the rule of method: facing what we cannot yet explain, it is wiser to admit our ignorance than to imagine causes to calm our unease.

Pierre-Simon de Laplace1872The famous anecdote is reported by Augustus De Morgan: when Napoleon reproached him for never naming the Creator, Laplace is said to have answered "I had no need of that hypothesis". It is a second-hand story, with no witness or date, which De Morgan calls "well known in Paris, but has never been printed entire": it appears in no book by Laplace. The diary of the astronomer William Herschel (August 1802) records a similar conversation, in which Laplace argues before Bonaparte that a chain of natural causes suffices to account for the system of the world, without that phrase.

In short

"I had no need of that hypothesis" is an argument by Pierre-Simon de Laplace (Exposition of the System of the World, book V, ch. VI and note VII (1835 ed.), 1835) on the question "Does God exist?" (answer: agnosticism). Its claim: In astronomy, the hypothesis of a divine design is not necessary. Where we do not yet know, it is wiser to admit our ignorance than to fill it with imagined causes. 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 2 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Exposition of the System of the World, book V, ch. VI and note VII (1835 ed.)

Valid with an implicit premise

Newton thought God must have arranged the solar system: all the planets turn in the same direction and nearly in the same plane, on nearly circular orbits, and he saw there an arrangement that mechanical causes could not produce. He also thought that, with time, the planets would drift and God would have to put them back in order. A century later, Laplace calculates the opposite: the arrangement of the planets is precisely the one that ensures stability. Then he suggests that this arrangement is itself the effect of the laws of motion, through the condensation of a nebula. He does not conclude that God does not exist. He concludes that astronomy does not need him to explain the heavens. A line attributed to Laplace before Napoleon made the idea famous.

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 case where the arrangement may be an effect of laws, where history shows gaps being filled, and where the arrangement nevertheless remains established as inexplicable by natural causes: the past does not bind the future, and a gap may resist.

The argument is valid with P5. P1 gives Newton's reasoning: God is required only if the arrangement is inexplicable or if a repair is necessary. P2 rules out the repair. P3 and P4 make the first case uncertain; P5 draws from them that the arrangement is not established as inexplicable. The conclusion is deliberately modest: it does not say God does not exist, but that he is not a necessary hypothesis of astronomy. The weak point is P1: a believer can answer that God is not a stopgap for gaps but the author of the laws themselves (see Paley's objection).

Analogy. You find a perfectly tidy room and conclude that someone tidied it. A physicist shows you that shaking the floor sorts objects by size on its own. You have not proved that nobody came: you no longer need to bring someone in to explain the tidiness. That is all Laplace says about the sky.

Objections

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

Isaac Newton (Opticks, as quoted by Laplace) 1704 · attacks premise 2

Newton writes that a blind fate could never have made all the planets move thus, apart from small inequalities. These inequalities, which come from the mutual action of planets and comets, will probably grow with time, until the system needs to be put back in order by its author. So the system will in the long run need its author to intervene.

a defender replies

Laplace defender of the argument · 1835 · Laplace's answer

Laplace answers that the conditions of the planets' arrangement are precisely those that ensure its stability: the motions are nearly circular, in the same direction, in planes that differ little, so the system only oscillates around a mean state. The mutual attraction of the bodies cannot alter its stability, contrary to what Newton supposed.

Laplace (a reservation of his own) 1835

Laplace does not claim that the system is eternal. He admits that the resistance of light, and the loss of the Sun's mass through its radiation, will in the long run destroy the arrangement of the planets, and that a repair would then become necessary. But he recalls that so many extinct animal species show a tendency of nature to change even the most fixed things, and that the solar system, vast for us, is only a point in the universe.

Where the debate stands (our summary)Our summary: on stability, the disagreement was settled in Laplace's favour for the perturbations of mutual attraction, which he could calculate. Laplace himself leaves open slower causes (resistance, loss of mass), which he judged irrelevant to the main point. The debate of substance is no longer about stability but about the origin of the arrangement (next objection).

William Paley 1802 · attacks premise 5

Paley holds that it is wrong to give a law as the cause of a phenomenon. For him, a law presupposes an agent, since it is only the way an agent proceeds. Without this agent, the law does nothing. Even if the solar system follows from laws such as those of the nebula, the question remains: who made these laws and these starting conditions? Explaining by laws does not remove the designer, it places him further back.

no reply

We have not yet found a direct reply by Laplace in the texts.

The debate goes on (our summary) Our summary: Laplace did not answer Paley, who writes after the first edition of the Exposition (1796) without quoting Laplace. But Laplace's text distinguishes two things: he refuses to fill a gap in science with a design (which is what Newton does), and he says nothing about the origin of the laws themselves. This distinction leaves Paley's question untouched, since it concerns the laws, not the gaps.

Where the debate stands (our summary)Our summary: the whole disagreement is about P1 and P5. Laplace treats God as a hypothesis to use when laws are missing; Paley sees him as what makes the laws possible. Laplace's argument shows that the "God of the gaps" retreats, but it does not settle the question of the "God of the laws". The debate is open.

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