Pascal's wager

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Blaise Pascal (1623–1662)

Pascal's wager is a philosophical argument advanced by Blaise Pascal (1623–1662), a French mathematician, philosopher, physicist, and theologian. This argument posits that individuals essentially engage in a life-defining gamble regarding the belief in the existence of God.

Pascal contends that a rational person should adopt a lifestyle consistent with the existence of God and should strive to believe in God. The reasoning for this stance involves the potential outcomes: if God does not exist, the believer incurs only finite losses, potentially sacrificing certain pleasures and luxuries; if God does exist, the believer stands to gain immeasurably, as represented for example by an eternity in Heaven in Abrahamic tradition, while simultaneously avoiding boundless losses associated with an eternity in Hell.

The first written expression of this wager is in Pascal's Pensées ("Thoughts"), a posthumous compilation of previously unpublished notes. Pascal's wager is the first formal application of decision theory, existentialism, pragmatism, and voluntarism.

Critics of the wager question the ability to provide definitive proof of God's existence. The argument from inconsistent revelations highlights the presence of various belief systems, each claiming exclusive access to divine truths. Additionally, the argument from inauthentic belief raises concerns about the genuineness of faith in God if it is motivated solely by potential benefits and losses.

The wager

The wager uses the following logic (excerpts from Pensées, part III, §233):

• "God is, or God is not. Reason cannot decide between the two alternatives"
• "A Game is being played ... where heads or tails will turn up"
• "You must wager; it is not optional"
• "Let us weigh the gain and the loss in wagering that God is. Let us estimate these two chances. If you gain, you gain all; if you lose, you lose nothing"
• "Wager, then, without hesitation that He is. ... There is here an infinity of an infinitely happy life to gain, a chance of gain against a finite number of chances of loss, and what you stake is finite. And so our proposition is of infinite force when there is the finite to stake in a game where there are equal risks of gain and of loss, and the infinite to gain."
• "But some cannot believe. They should then 'at least learn your inability to believe...' and 'Endeavour then to convince' themselves."

Pascal asks the reader to analyze humankind's position, where our actions can be enormously consequential, but our understanding of those consequences is flawed. While we can discern a great deal through reason, we are ultimately forced to gamble. Pascal cites a number of distinct areas of uncertainty in human life:

Category	Quotation(s)
Uncertainty in all	"This is what I see, and what troubles me. I look on all sides, and everywhere I see nothing but obscurity. Nature offers me nothing that is not a matter of doubt and disquiet."
Uncertainty in man's purpose	"For after all what is man in nature? A nothing in relation to infinity, all in relation to nothing, a central point between nothing and all and infinitely far from understanding either."
Uncertainty in reason	"There is nothing so conformable to reason as this disavowal of reason."
Uncertainty in science	"There is no doubt that natural laws exist, but once this fine reason of ours was corrupted, it corrupted everything."
Uncertainty in religion	"If I saw no signs of a divinity, I would fix myself in denial. If I saw everywhere the marks of a Creator, I would repose peacefully in faith. But seeing too much to deny Him, and too little to assure me, I am in a pitiful state, and I would wish a hundred times that if a god sustains nature it would reveal Him without ambiguity." "We understand nothing of the works of God unless we take it as a principle that He wishes to blind some and to enlighten others."
Uncertainty in skepticism	"It is not certain that everything is uncertain."

Pascal describes humanity as a finite being trapped within divine incomprehensibility, briefly thrust into being from non-being, with no explanation of "Why?" or "What?" or "How?" On Pascal's view, human finitude constrains our ability to achieve truth reliably.

Given that reason alone cannot determine whether God exists, Pascal concludes that this question functions as a coin toss. However, even if we do not know the outcome of this coin toss, we must base our actions on some expectation about the consequence. We must decide whether to live as though God exists, or whether to live as though God does not exist, even though we may be mistaken in either case.

In Pascal's assessment, participation in this wager is not optional. Merely by existing in a state of uncertainty, we are forced to choose between the available courses of action for practical purposes.

Pascal's description of the wager

The Pensées passage on Pascal's wager is as follows:

If there is a God, He is infinitely incomprehensible, since, having neither parts nor limits, He has no affinity to us. We are then incapable of knowing either what He is or if He is. ...

... "God is, or He is not." But to which side shall we incline? Reason can decide nothing here. There is infinite chaos that separated us. A game is being played at the extremity of this infinite distance where heads or tails will turn up. What will you wager? According to reason, you can do neither the one thing nor the other; according to reason, you can defend neither of the propositions.

Do not, then, reprove for error those who have made a choice; for you know nothing about it. "No, but I blame them for having made, not this choice, but a choice; for again both he who chooses heads and he who chooses tails are equally at fault, they are both in the wrong. The true course is not to wager at all."

Yes; but you must wager. It is not optional. You are embarked. Which will you choose then? Let us see. Since you must choose, let us see which interests you least. You have two things to lose, the true and the good; and two things to stake, your reason and your will, your knowledge and your happiness; and your nature has two things to shun, error and misery. Your reason is no more shocked in choosing one rather than the other since you must of necessity choose. This is one point settled. But your happiness? Let us weigh the gain and the loss in wagering that God is. Let us estimate these two chances. If you gain, you gain all; if you lose, you lose nothing. Wager, then, without hesitation that He is.

"That is very fine. Yes, I must wager; but I may perhaps wager too much." Let us see. Since there is an equal risk of gain and of loss, if you had only to gain two lives, instead of one, you might still wager. But if there were three lives to gain, you would have to play (since you are under the necessity of playing), and you would be imprudent, when you are forced to play, not to change your life to gain three at a game where there is an equal risk of loss and gain. But there is an eternity of life and happiness. And this being so, if there were an infinity of chances, of which one only would be for you, you would still be right in wagering one to win two, and you would act stupidly, being obliged to play, by refusing to stake one life against three at a game in which out of an infinity of chances there is one for you if there were an infinity of an infinitely happy life to gain. But there is here an infinity of an infinitely happy life to gain, a chance of gain against a finite number of chances of loss, and what you stake is finite.

Pascal begins by painting a situation where both the existence and non-existence of God are impossible to prove by human reason. So, supposing that reason cannot determine the truth between the two options, one must "wager" by weighing the possible consequences. Pascal's assumption is that, when it comes to making the decision, no one can refuse to participate; withholding assent is impossible because we are already "embarked", effectively living out the choice.

We only have two things to stake, our "reason" and our "happiness". Pascal considers that if there is "equal risk of loss and gain" (i.e. a coin toss), then human reason is powerless to address the question of whether God exists. That being the case, then human reason can only decide the question according to possible resulting happiness of the decision, weighing the gain and loss in believing that God exists and likewise in believing that God does not exist.

He points out that if a wager were between the equal chance of gaining two lifetimes of happiness and gaining nothing, then a person would be a fool to bet on the latter. The same would go if it were three lifetimes of happiness versus nothing. He then argues that it is simply unconscionable by comparison to bet against an eternal life of happiness for the possibility of gaining nothing. The wise decision is to wager that God exists, since "If you gain, you gain all; if you lose, you lose nothing", meaning one can gain eternal life if God exists, but if not, one will be no worse off in death than if one had not believed. On the other hand, if you bet against God, win or lose, you either gain nothing or lose everything. You are either unavoidably annihilated (in which case, nothing matters one way or the other) or miss the opportunity of eternal happiness. In note 194, speaking about those who live apathetically betting against God, he sums up by remarking, "It is to the glory of religion to have for enemies men so unreasonable".

Inability to believe

Pascal addressed the difficulty that reason and rationality pose to genuine belief by proposing that "acting as if [one] believed" could "cure [one] of unbelief":

But at least learn your inability to believe, since reason brings you to this, and yet you cannot believe. Endeavor then to convince yourself, not by increase of proofs of God, but by the abatement of your passions. You would like to attain faith, and do not know the way; you would like to cure yourself of unbelief and ask the remedy for it. Learn of those who have been bound like you, and who now stake all their possessions. These are people who know the way which you would follow, and who are cured of an ill of which you would be cured. Follow the way by which they began; by acting as if they believed, taking the holy water, having masses said, etc. Even this will naturally make you believe, and deaden your acuteness.

Analysis with decision theory

The possibilities defined by Pascal's wager can be thought of as a decision under uncertainty with the values of the following decision matrix.

	God exists (G)	God does not exist (¬G)
Belief (B)	+∞ (infinite gain)	−c (finite loss)
Disbelief (¬B)	−∞ (infinite loss)	+c (finite gain)

Given these values, the option of living as if God exists (B) dominates the option of living as if God does not exist (¬B), as long as one assumes a positive probability that God exists. In other words, the expected value gained by choosing B is greater than or equal to that of choosing ¬B.

In fact, according to decision theory, the only value that matters in the above matrix is the +∞ (infinitely positive). Any matrix of the following type (where f₁, f₂, and f₃ are all negative or finite positive numbers) results in (B) as being the only rational decision.

	God exists (G)	God does not exist (¬G)
Belief (B)	+∞	f1
Disbelief (¬B)	f2	f3

Misunderstanding of the wager

Pascal's intent was not to provide an argument to convince atheists to believe, but (a) to show the fallacy of attempting to use logical reasoning to prove or disprove God, and (b) to persuade atheists to sinlessness, as an aid to attaining faith ("it is this which will lessen the passions, which are your stumbling-blocks"). As Laurent Thirouin writes (note that the numbering of the items in the Pensees is not standardized; Thirouin's 418 is this article's 233):

The celebrity of fragment 418 has been established at the price of mutilation. By titling this text "the wager", readers have been fixated only on one part of Pascal's reasoning. It doesn't conclude with a QED at the end of the mathematical part. The unbeliever who had provoked this long analysis to counter his previous objection ("Maybe I bet too much") is still not ready to join the apologist on the side of faith. He put forward two new objections, undermining the foundations of the wager: the impossibility to know, and the obligation of playing.

To be put at the beginning of Pascal's planned book, the wager was meant to show that logical reasoning cannot support faith or lack thereof:

We have to accept reality and accept the reaction of the libertine when he rejects arguments he is unable to counter. The conclusion is evident: if men believe or refuse to believe, it is not how some believers sometimes say and most unbelievers claim because their own reason justifies the position they have adopted. Belief in God doesn't depend upon rational evidence, no matter which position.

Frederick Copleston writes that Pascal did not intend the wager as proof of God's existence or even a substitute for such proofs. He argues that the wager must be understood in the context of Pascal addressing the wager to those who "though they are also unconvinced by the arguments of sceptics and atheists" also "remain in a state of suspended judgment". Pascal's aim was to prepare "their minds and the production of dispositions favourable to belief".

Criticism

Criticism of Pascal's wager began soon after it was published. Non-believers questioned the "benefits" of a deity whose "realm" is beyond reason and the religiously orthodox, who primarily took issue with the wager's deistic and agnostic language. Believers criticized it for not proving God's existence, the encouragement of false belief, and the problem of which religion and which God should be worshipped.

Laplace

The probabilist mathematician Pierre Simon de Laplace ridiculed the use of probability in theology, believing that even following Pascal's reasoning, it is not worth making a bet, for the hope of profit – equal to the product of the value of the testimonies (infinitely small) and the value of the happiness they promise (which is significant but finite) – must necessarily be infinitely small.

Failure to prove the existence of God

Voltaire (another prominent French writer of the age of Enlightenment), a generation after Pascal, regarded the idea of the wager as a "proof of God" as "indecent and childish", adding, "the interest I have to believe a thing is no proof that such a thing exists". Pascal, however, did not advance the wager as a proof of God's existence but rather as a necessary pragmatic decision which is "impossible to avoid" for any living person. He argued that abstaining from making a wager is not an option and that "reason is incapable of divining the truth"; thus, a decision of whether to believe in the existence of God must be made by "considering the consequences of each possibility".

Voltaire's critique concerns not the nature of the Pascalian wager as proof of God's existence, but the contention that the very belief Pascal tried to promote is not convincing. Voltaire hints at the fact that Pascal, as a Jansenist, believed that only a small, and already predestined, portion of humanity would eventually be saved by God.

Voltaire explained that no matter how far someone is tempted with rewards to believe in Christian salvation, the result will be at best a faint belief. Pascal, in his Pensées, agrees with this, not stating that people can choose to believe (and therefore make a safe wager), but rather that some cannot believe.

As Étienne Souriau explained, in order to accept Pascal's argument, the bettor needs to be certain that God seriously intends to honour the bet; he says that the wager assumes that God also accepts the bet, which is not proved; Pascal's bettor is here like the fool who seeing a leaf floating on a river's waters and quivering at some point, for a few seconds, between the two sides of a stone, says: "I bet a million with Rothschild that it takes finally the left path." And, effectively, the leaf passed on the left side of the stone, but unfortunately for the fool Rothschild never said "I [will take that] bet".

Argument from inconsistent revelations

Main article: Argument from inconsistent revelations

Since there have been many religions throughout history, and therefore many conceptions of God (or gods), some assert that all of them need to be factored into the wager, in an argumentation known as the argument from inconsistent revelations. This, its proponents argue, would lead to a high probability of believing in "the wrong god" and would eliminate the mathematical advantage Pascal claimed with his wager. Denis Diderot, a contemporary of Voltaire, expressed this opinion when asked about the wager, saying "an Imam could reason the same way". J. L. Mackie writes that "the church within which alone salvation is to be found is not necessarily the Church of Rome, but perhaps that of the Anabaptists or the Mormons or the Muslim Sunnis or the worshipers of Kali or of Odin."

Pascal considers this type of objection briefly in the notes compiled into the Pensées, and dismisses it:

What say [the unbelievers] then? "Do we not see," say they, "that the brutes live and die like men, and Turks like Christians? They have their ceremonies, their prophets, their doctors, their saints, their monks, like us," etc. If you care but little to know the truth, that is enough to leave you in repose. But if you desire with all your heart to know it, it is not enough; look at it in detail. That would be sufficient for a question in philosophy; but not here, where everything is at stake. And yet, after a superficial reflection of this kind, we go to amuse ourselves, etc. Let us inquire of this same religion whether it does not give a reason for this obscurity; perhaps it will teach it to us.

Pascal says that the skepticism of unbelievers who rest content with the many-religions objection has seduced them into a fatal "repose". If they were really bent on knowing the truth, they would be persuaded to examine "in detail" whether Christianity is like any other religion, but they just cannot be bothered. Their objection might be sufficient were the subject concerned merely some "question in philosophy", but not "here, where everything is at stake". In "a matter where they themselves, their eternity, their all are concerned", they can manage no better than "a superficial reflection" ("une reflexion légère") and, thinking they have scored a point by asking a leading question, they go off to amuse themselves.

As Pascal scholars observe, Pascal regarded the many-religions objection as a rhetorical ploy, a "trap" that he had no intention of falling into.

David Wetsel notes that Pascal's treatment of the pagan religions is brisk: "As far as Pascal is concerned, the demise of the pagan religions of antiquity speaks for itself. Those pagan religions which still exist in the New World, in India, and in Africa are not even worth a second glance. They are obviously the work of superstition and ignorance and have nothing in them which might interest "les gens habiles" ('clever men'). Islam warrants more attention, being distinguished from paganism (which for Pascal presumably includes all the other non-Christian religions) by its claim to be a revealed religion. Nevertheless, Pascal concludes that the religion founded by Mohammed can on several counts be shown to be devoid of divine authority, and that therefore, as a path to the knowledge of God, it is as much a dead end as paganism." Judaism, in view of its close links to Christianity, he deals with elsewhere.

The many-religions objection is taken more seriously by some later apologists of the wager, who argue that of the rival options only those awarding infinite happiness affect the wager's dominance. In the opinion of these apologists "finite, semi-blissful promises such as Kali's or Odin's" therefore drop out of consideration. Also, the infinite bliss that the rival conception of God offers has to be mutually exclusive. If Christ's promise of bliss can be attained concurrently with Jehovah's and Allah's (all three being identified as the God of Abraham), there is no conflict in the decision matrix in the case where the cost of believing in the wrong conception of God is neutral (limbo/purgatory/spiritual death), although this would be countered with an infinite cost in the case where not believing in the correct conception of God results in punishment (hell).

Ecumenical interpretations of the wager argue that it could even be suggested that believing in a generic God, or a god by the wrong name, is acceptable so long as that conception of God has similar essential characteristics of the conception of God considered in Pascal's wager (perhaps the God of Aristotle). Proponents of this line of reasoning suggest that either all of the conceptions of God or gods throughout history truly boil down to just a small set of "genuine options", or that if Pascal's wager can simply bring a person to believe in "generic theism", it has done its job.

Pascal argues implicitly for the uniqueness of Christianity in the wager itself, writing: "If there is a God, He is infinitely incomprehensible ... Who then can blame the Christians for not being able to give reasons for their beliefs, professing as they do a religion which they cannot explain by reason?"

Argument from inauthentic belief

Some critics argue that Pascal's wager, for those who cannot believe, suggests feigning belief to gain eternal reward. Richard Dawkins argues that this would be dishonest and immoral and that, in addition to this, it is absurd to think that God, being just and omniscient, would not see through this deceptive strategy on the part of the "believer", thus nullifying the benefits of the wager. William James in his 'Will to Believe' states that "We feel that a faith in masses and holy water adopted wilfully after such a mechanical calculation would lack the inner soul of faith's reality; and if we were ourselves in the place of the Deity, we should probably take particular pleasure in cutting off believers of this pattern from their infinite reward. It is evident that unless there be some pre-existing tendency to believe in masses and holy water, the option offered to the will by Pascal is not a living option".

Since these criticisms are concerned not with the validity of the wager itself, but with its possible aftermath—namely that a person who has been convinced of the overwhelming odds in favor of belief might still find themself unable to sincerely believe—they are tangential to the thrust of the wager. What such critics are objecting to is Pascal's subsequent advice to an unbeliever who, having concluded that the only rational way to wager is in favor of God's existence, points out, reasonably enough, that this by no means makes them a believer. This hypothetical unbeliever complains, "I am so made that I cannot believe. What would you have me do?" Pascal, far from suggesting that God can be deceived by outward show, says that God does not regard it at all: "God looks only at what is inward." For a person who is already convinced of the odds of the wager but cannot seem to put their heart into the belief, he offers practical advice.

Explicitly addressing the question of inability to believe, Pascal argues that if the wager is valid, the inability to believe is irrational, and therefore must be caused by feelings: "your inability to believe, because reason compels you to [believe] and yet you cannot, [comes] from your passions." This inability, therefore, can be overcome by diminishing these irrational sentiments: "Learn from those who were bound like you. ... Follow the way by which they began; by acting as if they believed, taking the holy water, having masses said, etc. Even this will naturally make you believe, and deaden your acuteness.—'But this is what I am afraid of.'—And why? What have you to lose?"

An uncontroversial doctrine in both Roman Catholic and Protestant theology is that mere belief in God is insufficient to attain salvation, the standard cite being James 2:19 (the following is from the KJV): "Thou believest that there is one God; thou doest well: the devils also believe, and tremble." Salvation requires "faith" not just in the sense of belief, but of trust and obedience. Pascal and his sister, a nun, were among the leaders of Roman Catholicism's Jansenist school of thought whose doctrine of salvation was close to Protestantism in emphasizing faith over works. Both Jansenists and Protestants followed St. Augustine in this emphasis (Martin Luther belonged to the Augustinian Order of monks). Augustine wrote

So our faith has to be distinguished from the faith of the demons. Our faith, you see, purifies the heart, their faith makes them guilty. They act wickedly, and so they say to the Lord, "What have you to do with us?" When you hear the demons saying this, do you imagine they don't recognize him? "We know who you are," they say. "You are the Son of God" (Lk 4:34). Peter says this and he is praised for it; the demon says it, and is condemned. Why's that, if not because the words may be the same, but the heart is very different? So let us distinguish our faith, and see that believing is not enough. That's not the sort of faith that purifies the heart.

Since Pascal's position was that "saving" belief in God required more than logical assent, accepting the wager could only be a first step. Hence his advice on what steps one could take to arrive at belief.

Some other critics have objected to Pascal's wager on the grounds that he wrongly assumes what type of epistemic character God would likely value in his rational creatures if he existed.

Earlier versions and other wager arguments

• The sophist Protagoras had an agnostic position regarding the gods, but he nevertheless continued to worship the gods. This could be considered as an early version of the Wager.
• In the famous tragedy of Euripides Bacchae, Kadmos states an early version of Pascal's wager. It is noteworthy that at the end of the tragedy Dionysos, the god to whom Kadmos referred, appears and punishes him for thinking in this way. Euripides, quite clearly, considered and dismissed the wager in this tragedy.
• The stoic philosopher and Roman Emperor Marcus Aurelius expressed a similar sentiment in the second book of Meditations, saying "Since it is possible that thou mayest depart from life this very moment, regulate every act and thought accordingly. But to go away from among men, if there are gods, is not a thing to be afraid of, for the gods will not involve thee in evil; but if indeed they do not exist, or if they have no concern about human affairs, what is it to me to live in a universe devoid of gods or devoid of Providence?"
• The early Buddhist texts contain passages which defend a Buddhist wager argument for believing in an afterlife.
• In the Sanskrit classic Sārasamuccaya, Vararuci makes a similar argument to Pascal's wager.
• Muslim Imam Ja'far al-Sadiq is recorded to have postulated variations of the wager on several occasions in different forms, including his famed 'Tradition of the Myrobalan Fruit.' In the Shi'i hadith book al-Kafi, al-Sadiq declares to an atheist "If what you say is correct – and it is not – then we will both succeed. But if what I say is correct – and it is – then I will succeed, and you will be destroyed."
• An instantiation of this argument, within the Islamic kalam tradition, was discussed by Imam al-Haramayn al-Juwayni (d. 478/1085) in his Kitab al-irshad ila-qawati al-adilla fi usul al-i'tiqad, or A Guide to the Conclusive Proofs for the Principles of Belief.
• The Christian apologist Arnobius of Sicca (d. 330) stated an early version of the argument in his book Against the Pagans, arguing "is it not more rational, of two things uncertain and hanging in doubtful suspense, rather to believe that which carries with it some hopes, than that which brings none at all?"
• A close parallel just before Pascal's time occurred in the Jesuit Antoine Sirmond's On the Immortality of the Soul (1635), which explicitly compared the choice of religion to playing dice and argued "However long and happy the space of this life may be, while ever you place it in the other pan of the balance against a blessed and flourishing eternity, surely it will seem to you ... that the pan will rise on high."
• The Atheist's Wager, popularised by the philosopher Michael Martin and published in his 1990 book Atheism: A Philosophical Justification, is an atheistic wager argument in response to Pascal's wager.
• A 2008 philosophy book, How to Make Good Decisions and Be Right All the Time, presents a secular revision of Pascal's wager: "What does it hurt to pursue value and virtue? If there is value, then we have everything to gain, but if there is none, then we haven’t lost anything.... Thus, we should seek value."
• Pascal's mugging, a dialogue written by philosopher Nick Bostrom, shows that a rational victim can be made to give up his wallet in exchange for a weakly credible promise of astronomical repayment. As in Pascal's Wager, a small but certain downside is outweighed by a large but unlikely upside.
• Roko's basilisk is a hypothetical future superintelligence that punishes everyone who failed to help bring it into existence.
• In a 2014 article, philosopher Justin McBrayer argued we ought to remain agnostic about the existence of God but nonetheless believe because of the good that comes in the present life from believing in God. "The gist of the renewed wager is that theists do better than non-theists regardless of whether or not God exists."

Climate change

Since at least 1992, some scholars have analogized Pascal's wager to decisions about climate change. Two differences from Pascal's wager are posited regarding climate change: first, climate change is more likely than Pascal's God to exist, as there is scientific evidence for one but not the other. Secondly, the calculated penalty for unchecked climate change would be large, but is not generally considered to be infinite. Magnate Warren Buffett has written that climate change "bears a similarity to Pascal's Wager on the Existence of God. Pascal, it may be recalled, argued that if there were only a tiny probability that God truly existed, it made sense to behave as if He did because the rewards could be infinite whereas the lack of belief risked eternal misery. Likewise, if there is only a 1% chance the planet is heading toward a truly major disaster and delay means passing a point of no return, inaction now is foolhardy."

Pierre-Simon Laplace

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Pierre-Simon_Laplace

 

Pierre-Simon Laplace
Pierre-Simon Laplace as chancellor of the Senate under the First French Empire
Born	23 March 1749 Beaumont-en-Auge, Normandy, Kingdom of France
Died	5 March 1827 (aged 77) Paris, Kingdom of France
Alma mater	University of Caen
Known for

Scientific career
Fields	Astronomy and Mathematics
Institutions	École Militaire (1769–1776)
Academic advisors	Jean d'Alembert Christophe Gadbled Pierre Le Canu
Notable students	Siméon Denis Poisson Napoleon Bonaparte
Minister of the Interior
In office 12 November 1799 – 25 December 1799
Prime Minister	Napoleon Bonaparte (as First Consul)
Preceded by	Nicolas Marie Quinette
Succeeded by	Lucien Bonaparte
Signature

Pierre-Simon, Marquis de Laplace (/ləˈplɑːs/; French: [pjɛʁ simɔ̃ laplas]; 23 March 1749 – 5 March 1827) was a French polymath, a scholar whose work has been instrumental in the fields of physics, astronomy, mathematics, engineering, statistics, and philosophy. He summarized and extended the work of his predecessors in his five-volume Mécanique céleste (Celestial Mechanics) (1799–1825). This work translated the geometric study of classical mechanics to one based on calculus, opening up a broader range of problems. Laplace also popularized and further confirmed Sir Isaac Newton's work. In statistics, the Bayesian interpretation of probability was developed mainly by Laplace.

Laplace formulated Laplace's equation, and pioneered the Laplace transform which appears in many branches of mathematical physics, a field that he took a leading role in forming. The Laplacian differential operator, widely used in mathematics, is also named after him. He restated and developed the nebular hypothesis of the origin of the Solar System and was one of the first scientists to suggest an idea similar to that of a black hole, with Stephen Hawking stating that "Laplace essentially predicted the existence of black holes". He originated Laplace's demon, which is a hypothetical all-predicting intellect. He also refined Newton's calculation of the speed of sound to derive a more accurate measurement.

Laplace is regarded as one of the greatest scientists of all time. Sometimes referred to as the French Newton or Newton of France, he has been described as possessing a phenomenal natural mathematical faculty superior to that of almost all of his contemporaries. He was Napoleon's examiner when Napoleon graduated from the École Militaire in Paris in 1785. Laplace became a count of the Empire in 1806 and was named a marquis in 1817, after the Bourbon Restoration.

Early years

Portrait of Pierre-Simon Laplace by Johann Ernst Heinsius (1775)

Some details of Laplace's life are not known, as records of it were burned in 1925 with the family château in Saint Julien de Mailloc, near Lisieux, the home of his great-great-grandson the Comte de Colbert-Laplace. Others had been destroyed earlier, when his house at Arcueil near Paris was looted in 1871.

Laplace was born in Beaumont-en-Auge, Normandy on 23 March 1749, a village four miles west of Pont l'Évêque. According to W. W. Rouse Ball, his father, Pierre de Laplace, owned and farmed the small estates of Maarquis. His great-uncle, Maitre Oliver de Laplace, had held the title of Chirurgien Royal. It would seem that from a pupil he became an usher in the school at Beaumont; but, having procured a letter of introduction to d'Alembert, he went to Paris to advance his fortune. However, Karl Pearson is scathing about the inaccuracies in Rouse Ball's account and states:

Indeed Caen was probably in Laplace's day the most intellectually active of all the towns of Normandy. It was here that Laplace was educated and was provisionally a professor. It was here he wrote his first paper published in the Mélanges of the Royal Society of Turin, Tome iv. 1766–1769, at least two years before he went at 22 or 23 to Paris in 1771. Thus before he was 20 he was in touch with Lagrange in Turin. He did not go to Paris a raw self-taught country lad with only a peasant background! In 1765 at the age of sixteen Laplace left the "School of the Duke of Orleans" in Beaumont and went to the University of Caen, where he appears to have studied for five years and was a member of the Sphinx. The École Militaire of Beaumont did not replace the old school until 1776.

His parents, Pierre Laplace and Marie-Anne Sochon, were from comfortable families. The Laplace family was involved in agriculture until at least 1750, but Pierre Laplace senior was also a cider merchant and syndic of the town of Beaumont.

Pierre Simon Laplace attended a school in the village run at a Benedictine priory, his father intending that he be ordained in the Roman Catholic Church. At sixteen, to further his father's intention, he was sent to the University of Caen to read theology.

At the university, he was mentored by two enthusiastic teachers of mathematics, Christophe Gadbled and Pierre Le Canu, who awoke his zeal for the subject. Here Laplace's brilliance as a mathematician was quickly recognised and while still at Caen he wrote a memoir Sur le Calcul integral aux differences infiniment petites et aux differences finies. This provided the first correspondence between Laplace and Lagrange. Lagrange was the senior by thirteen years, and had recently founded in his native city Turin a journal named Miscellanea Taurinensia, in which many of his early works were printed and it was in the fourth volume of this series that Laplace's paper appeared. About this time, recognising that he had no vocation for the priesthood, he resolved to become a professional mathematician. Some sources state that he then broke with the church and became an atheist. Laplace did not graduate in theology but left for Paris with a letter of introduction from Le Canu to Jean le Rond d'Alembert who at that time was supreme in scientific circles.

According to his great-great-grandson, d'Alembert received him rather poorly, and to get rid of him gave him a thick mathematics book, saying to come back when he had read it. When Laplace came back a few days later, d'Alembert was even less friendly and did not hide his opinion that it was impossible that Laplace could have read and understood the book. But upon questioning him, he realised that it was true, and from that time he took Laplace under his care.

Another account is that Laplace solved overnight a problem that d'Alembert set him for submission the following week, then solved a harder problem the following night. D'Alembert was impressed and recommended him for a teaching place in the École Militaire.

With a secure income and undemanding teaching, Laplace now threw himself into original research and for the next seventeen years, 1771–1787, he produced much of his original work in astronomy.

The Calorimeter of Lavoisier and La Place, Encyclopaedia Londinensis, 1801

From 1780 to 1784, Laplace and French chemist Antoine Lavoisier collaborated on several experimental investigations, designing their own equipment for the task. In 1783 they published their joint paper, Memoir on Heat, in which they discussed the kinetic theory of molecular motion. In their experiments they measured the specific heat of various bodies, and the expansion of metals with increasing temperature. They also measured the boiling points of ethanol and ether under pressure.

Laplace further impressed the Marquis de Condorcet, and already by 1771 Laplace felt entitled to membership in the French Academy of Sciences. However, that year admission went to Alexandre-Théophile Vandermonde and in 1772 to Jacques Antoine Joseph Cousin. Laplace was disgruntled, and early in 1773 d'Alembert wrote to Lagrange in Berlin to ask if a position could be found for Laplace there. However, Condorcet became permanent secretary of the Académie in February and Laplace was elected associate member on 31 March, at age 24. In 1773 Laplace read his paper on the invariability of planetary motion in front of the Academy des Sciences. That March he was elected to the academy, a place where he conducted the majority of his science.

On 15 March 1788, at the age of thirty-nine, Laplace married Marie-Charlotte de Courty de Romanges, an eighteen-year-old girl from a "good" family in Besançon. The wedding was celebrated at Saint-Sulpice, Paris. The couple had a son, Charles-Émile (1789–1874), and a daughter, Sophie-Suzanne (1792–1813).

Analysis, probability, and astronomical stability

Laplace's early published work in 1771 started with differential equations and finite differences but he was already starting to think about the mathematical and philosophical concepts of probability and statistics. However, before his election to the Académie in 1773, he had already drafted two papers that would establish his reputation. The first, Mémoire sur la probabilité des causes par les événements was ultimately published in 1774 while the second paper, published in 1776, further elaborated his statistical thinking and also began his systematic work on celestial mechanics and the stability of the Solar System. The two disciplines would always be interlinked in his mind. "Laplace took probability as an instrument for repairing defects in knowledge." Laplace's work on probability and statistics is discussed below with his mature work on the analytic theory of probabilities.

Stability of the Solar System

Sir Isaac Newton had published his Philosophiæ Naturalis Principia Mathematica in 1687 in which he gave a derivation of Kepler's laws, which describe the motion of the planets, from his laws of motion and his law of universal gravitation. However, though Newton had privately developed the methods of calculus, all his published work used cumbersome geometric reasoning, unsuitable to account for the more subtle higher-order effects of interactions between the planets. Newton himself had doubted the possibility of a mathematical solution to the whole, even concluding that periodic divine intervention was necessary to guarantee the stability of the Solar System. Dispensing with the hypothesis of divine intervention would be a major activity of Laplace's scientific life. It is now generally regarded that Laplace's methods on their own, though vital to the development of the theory, are not sufficiently precise to demonstrate the stability of the Solar System; today the Solar System is understood to be generally chaotic at fine scales, although currently fairly stable on coarse scale.

One particular problem from observational astronomy was the apparent instability whereby Jupiter's orbit appeared to be shrinking while that of Saturn was expanding. The problem had been tackled by Leonhard Euler in 1748, and Joseph Louis Lagrange in 1763, but without success. In 1776, Laplace published a memoir in which he first explored the possible influences of a purported luminiferous ether or of a law of gravitation that did not act instantaneously. He ultimately returned to an intellectual investment in Newtonian gravity. Euler and Lagrange had made a practical approximation by ignoring small terms in the equations of motion. Laplace noted that though the terms themselves were small, when integrated over time they could become important. Laplace carried his analysis into the higher-order terms, up to and including the cubic. Using this more exact analysis, Laplace concluded that any two planets and the Sun must be in mutual equilibrium and thereby launched his work on the stability of the Solar System. Gerald James Whitrow described the achievement as "the most important advance in physical astronomy since Newton".

Laplace had a wide knowledge of all sciences and dominated all discussions in the Académie. Laplace seems to have regarded analysis merely as a means of attacking physical problems, though the ability with which he invented the necessary analysis is almost phenomenal. As long as his results were true he took but little trouble to explain the steps by which he arrived at them; he never studied elegance or symmetry in his processes, and it was sufficient for him if he could by any means solve the particular question he was discussing.

Tidal dynamics

Main article: Theory of tides

Dynamic theory of tides

While Newton explained the tides by describing the tide-generating forces and Bernoulli gave a description of the static reaction of the waters on Earth to the tidal potential, the dynamic theory of tides, developed by Laplace in 1775, describes the ocean's real reaction to tidal forces. Laplace's theory of ocean tides took into account friction, resonance and natural periods of ocean basins. It predicted the large amphidromic systems in the world's ocean basins and explains the oceanic tides that are actually observed.

The equilibrium theory, based on the gravitational gradient from the Sun and Moon but ignoring the Earth's rotation, the effects of continents, and other important effects, could not explain the real ocean tides.

Since measurements have confirmed the theory, many things have possible explanations now, like how the tides interact with deep sea ridges and chains of seamounts give rise to deep eddies that transport nutrients from the deep to the surface. The equilibrium tide theory calculates the height of the tide wave of less than half a meter, while the dynamic theory explains why tides are up to 15 meters. Satellite observations confirm the accuracy of the dynamic theory, and the tides worldwide are now measured to within a few centimeters. Measurements from the CHAMP satellite closely match the models based on the TOPEX data. Accurate models of tides worldwide are essential for research since the variations due to tides must be removed from measurements when calculating gravity and changes in sea levels.

Laplace's tidal equations

A. Lunar gravitational potential: this depicts the Moon directly over 30° N (or 30° S) viewed from above the Northern Hemisphere.

B. This view shows same potential from 180° from view A. Viewed from above the Northern Hemisphere. Red up, blue down.

In 1776, Laplace formulated a single set of linear partial differential equations, for tidal flow described as a barotropic two-dimensional sheet flow. Coriolis effects are introduced as well as lateral forcing by gravity. Laplace obtained these equations by simplifying the fluid dynamic equations. But they can also be derived from energy integrals via Lagrange's equation.

For a fluid sheet of average thickness D, the vertical tidal elevation ζ, as well as the horizontal velocity components u and v (in the latitude φ and longitude λ directions, respectively) satisfy Laplace's tidal equations:

${\displaystyle \begin{array}{rl}\frac{\mathrm{\partial }\zeta }{\mathrm{\partial }t}& +\frac{1}{a\cos (\phi )}\left[\frac{\mathrm{\partial }}{\mathrm{\partial }\lambda }(uD)+\frac{\mathrm{\partial }}{\mathrm{\partial }\phi }\left(vD\cos (\phi )\right)\right]=0,\\ \frac{\mathrm{\partial }u}{\mathrm{\partial }t}& -v\left(2\mathrm{\Omega }\sin (\phi )\right)+\frac{1}{a\cos (\phi )}\frac{\mathrm{\partial }}{\mathrm{\partial }\lambda }\left(g\zeta +U\right)=0\text{and}\\ \frac{\mathrm{\partial }v}{\mathrm{\partial }t}& +u\left(2\mathrm{\Omega }\sin (\phi )\right)+\frac{1}{a}\frac{\mathrm{\partial }}{\mathrm{\partial }\phi }\left(g\zeta +U\right)=0,\end{array}}$

where Ω is the angular frequency of the planet's rotation, g is the planet's gravitational acceleration at the mean ocean surface, a is the planetary radius, and U is the external gravitational tidal-forcing potential.

William Thomson (Lord Kelvin) rewrote Laplace's momentum terms using the curl to find an equation for vorticity. Under certain conditions this can be further rewritten as a conservation of vorticity.

On the figure of the Earth

During the years 1784–1787 he published some papers of exceptional power. Prominent among these is one read in 1783, reprinted as Part II of Théorie du Mouvement et de la figure elliptique des planètes in 1784, and in the third volume of the Mécanique céleste. In this work, Laplace completely determined the attraction of a spheroid on a particle outside it. This is memorable for the introduction into analysis of spherical harmonics or Laplace's coefficients, and also for the development of the use of what we would now call the gravitational potential in celestial mechanics.

Spherical harmonics

Spherical harmonics

In 1783, in a paper sent to the Académie, Adrien-Marie Legendre had introduced what are now known as associated Legendre functions. If two points in a plane have polar coordinates (r, θ) and (r ', θ'), where r ' ≥ r, then, by elementary manipulation, the reciprocal of the distance between the points, d, can be written as:

${\displaystyle \frac{1}{d}=\frac{1}{r^{\prime }}{\left[1-2\cos ({\theta }^{\prime }-\theta )\frac{r}{r^{\prime }}+{\left(\frac{r}{r^{\prime }}\right)}^2\right]}^{-\frac{1}{2}}.}$

This expression can be expanded in powers of r/r ' using Newton's generalised binomial theorem to give:

${\displaystyle \frac{1}{d}=\frac{1}{r^{\prime }}\sum _{k=0}^{\mathrm{\infty }}{P}_k^0(\cos ({\theta }^{\prime }-\theta )){\left(\frac{r}{r^{\prime }}\right)}^k.}$

The sequence of functions P⁰_k(cos φ) is the set of so-called "associated Legendre functions" and their usefulness arises from the fact that every function of the points on a circle can be expanded as a series of them.

Laplace, with scant regard for credit to Legendre, made the non-trivial extension of the result to three dimensions to yield a more general set of functions, the spherical harmonics or Laplace coefficients. The latter term is not in common use now.

Potential theory

This paper is also remarkable for the development of the idea of the scalar potential. The gravitational force acting on a body is, in modern language, a vector, having magnitude and direction. A potential function is a scalar function that defines how the vectors will behave. A scalar function is computationally and conceptually easier to deal with than a vector function.

Alexis Clairaut had first suggested the idea in 1743 while working on a similar problem though he was using Newtonian-type geometric reasoning. Laplace described Clairaut's work as being "in the class of the most beautiful mathematical productions". However, Rouse Ball alleges that the idea "was appropriated from Joseph Louis Lagrange, who had used it in his memoirs of 1773, 1777 and 1780". The term "potential" itself was due to Daniel Bernoulli, who introduced it in his 1738 memoire Hydrodynamica. However, according to Rouse Ball, the term "potential function" was not actually used (to refer to a function V of the coordinates of space in Laplace's sense) until George Green's 1828 An Essay on the Application of Mathematical Analysis to the Theories of Electricity and Magnetism.

Laplace applied the language of calculus to the potential function and showed that it always satisfies the differential equation:

${\displaystyle {\mathrm{\nabla }}^{2}V=\frac{{\mathrm{\partial }}^{2}V}{\mathrm{\partial }{x}^2}+\frac{{\mathrm{\partial }}^{2}V}{\mathrm{\partial }{y}^2}+\frac{{\mathrm{\partial }}^{2}V}{\mathrm{\partial }{z}^2}=0.}$

An analogous result for the velocity potential of a fluid had been obtained some years previously by Leonhard Euler.

Laplace's subsequent work on gravitational attraction was based on this result. The quantity ∇²V has been termed the concentration of V and its value at any point indicates the "excess" of the value of V there over its mean value in the neighbourhood of the point. Laplace's equation, a special case of Poisson's equation, appears ubiquitously in mathematical physics. The concept of a potential occurs in fluid dynamics, electromagnetism and other areas. Rouse Ball speculated that it might be seen as "the outward sign" of one of the a priori forms in Kant's theory of perception.

The spherical harmonics turn out to be critical to practical solutions of Laplace's equation. Laplace's equation in spherical coordinates, such as are used for mapping the sky, can be simplified, using the method of separation of variables into a radial part, depending solely on distance from the centre point, and an angular or spherical part. The solution to the spherical part of the equation can be expressed as a series of Laplace's spherical harmonics, simplifying practical computation.

Planetary and lunar inequalities

Title page of an 1817 copy of Delambre's "Tables écliptiques des satellites de Jupiter", which references Laplace's contributions in its title

Tables in an 1817 copy of Delambre's "Tables écliptiques des satellites de Jupiter" – these calculations were influenced by Laplace's previous discoveries.

Jupiter–Saturn great inequality

Laplace presented a memoir on planetary inequalities in three sections, in 1784, 1785, and 1786. This dealt mainly with the identification and explanation of the perturbations now known as the "great Jupiter–Saturn inequality". Laplace solved a longstanding problem in the study and prediction of the movements of these planets. He showed by general considerations, first, that the mutual action of two planets could never cause large changes in the eccentricities and inclinations of their orbits; but then, even more importantly, that peculiarities arose in the Jupiter–Saturn system because of the near approach to commensurability of the mean motions of Jupiter and Saturn.

In this context commensurability means that the ratio of the two planets' mean motions is very nearly equal to a ratio between a pair of small whole numbers. Two periods of Saturn's orbit around the Sun almost equal five of Jupiter's. The corresponding difference between multiples of the mean motions, (2n_J − 5n_S), corresponds to a period of nearly 900 years, and it occurs as a small divisor in the integration of a very small perturbing force with this same period. As a result, the integrated perturbations with this period are disproportionately large, about 0.8° degrees of arc in orbital longitude for Saturn and about 0.3° for Jupiter.

Further developments of these theorems on planetary motion were given in his two memoirs of 1788 and 1789, but with the aid of Laplace's discoveries, the tables of the motions of Jupiter and Saturn could at last be made much more accurate. It was on the basis of Laplace's theory that Delambre computed his astronomical tables.

Books

Laplace now set himself the task to write a work which should "offer a complete solution of the great mechanical problem presented by the Solar System, and bring theory to coincide so closely with observation that empirical equations should no longer find a place in astronomical tables." The result is embodied in the Exposition du système du monde and the Mécanique céleste.

The former was published in 1796, and gives a general explanation of the phenomena, but omits all details. It contains a summary of the history of astronomy. This summary procured for its author the honour of admission to the forty of the French Academy and is commonly esteemed one of the masterpieces of French literature, though it is not altogether reliable for the later periods of which it treats.

Laplace developed the nebular hypothesis of the formation of the Solar System, first suggested by Emanuel Swedenborg and expanded by Immanuel Kant. This hypothesis remains the most widely accepted model in the study of the origin of planetary systems. According to Laplace's description of the hypothesis, the Solar System evolved from a globular mass of incandescent gas rotating around an axis through its centre of mass. As it cooled, this mass contracted, and successive rings broke off from its outer edge. These rings in their turn cooled, and finally condensed into the planets, while the Sun represented the central core which was still left. On this view, Laplace predicted that the more distant planets would be older than those nearer the Sun.

As mentioned, the idea of the nebular hypothesis had been outlined by Immanuel Kant in 1755, who had also suggested "meteoric aggregations" and tidal friction as causes affecting the formation of the Solar System. Laplace was probably aware of this, but, like many writers of his time, he generally did not reference the work of others.

Laplace's analytical discussion of the Solar System is given in his Mécanique céleste published in five volumes. The first two volumes, published in 1799, contain methods for calculating the motions of the planets, determining their figures, and resolving tidal problems. The third and fourth volumes, published in 1802 and 1805, contain applications of these methods, and several astronomical tables. The fifth volume, published in 1825, is mainly historical, but it gives as appendices the results of Laplace's latest researches. The Mécanique céleste contains numerous of Laplace's own investigations but many results are appropriated from other writers with little or no acknowledgement. The volume's conclusions, which are described by historians as the organised result of a century of work by other writers as well as Laplace, are presented by Laplace as if they were his discoveries alone.

First pages to Exposition du Système du Monde (1799)

Jean-Baptiste Biot, who assisted Laplace in revising it for the press, says that Laplace himself was frequently unable to recover the details in the chain of reasoning, and, if satisfied that the conclusions were correct, he was content to insert the phrase, "Il est aisé à voir que..." ("It is easy to see that..."). The Mécanique céleste is not only the translation of Newton's Principia Mathematica into the language of differential calculus, but it completes parts of which Newton had been unable to fill in the details. The work was carried forward in a more finely tuned form in Félix Tisserand's Traité de mécanique céleste (1889–1896), but Laplace's treatise remains a standard authority. In the years 1784–1787, Laplace produced some memoirs of exceptional power. The significant among these was one issued in 1784, and reprinted in the third volume of the Mécanique céleste. In this work he completely determined the attraction of a spheroid on a particle outside it. This is known for the introduction into analysis of the potential, a useful mathematical concept of broad applicability to the physical sciences.

Optics

Laplace was a supporter of the corpuscle theory of light of Newton. In the fourth edition of Mécanique Céleste, Laplace assumed that short-ranged molecular forces were responsible for refraction of the corpuscles of light. Laplace and Étienne-Louis Malus also showed that Huygens principle of double refraction could be recovered from the principle of least action on light particles.

However in 1815, Augustin-Jean Fresnel presented a new wave theory for diffraction to a commission of the French Academy with the help of François Arago. Laplace was one of the commission members and they ultimately awarded a prize to Fresnel for his new approach.

Influence of gravity on light

Using corpuscular theory, Laplace also came close to propounding the concept of the black hole. He suggested that gravity could influence light and that there could be massive stars whose gravity is so great that not even light could escape from their surface (see escape velocity). However, this insight was so far ahead of its time that it played no role in the history of scientific development.

Arcueil

Laplace's house at Arcueil to the south of Paris

Main article: Society of Arcueil

In 1806, Laplace bought a house in Arcueil, then a village and not yet absorbed into the Paris conurbation. The chemist Claude Louis Berthollet was a neighbour – their gardens were not separated – and the pair formed the nucleus of an informal scientific circle, latterly known as the Society of Arcueil. Because of their closeness to Napoleon, Laplace and Berthollet effectively controlled advancement in the scientific establishment and admission to the more prestigious offices. The Society built up a complex pyramid of patronage. In 1806, Laplace was also elected a foreign member of the Royal Swedish Academy of Sciences.

Analytic theory of probabilities

In 1812, Laplace issued his Théorie analytique des probabilités in which he laid down many fundamental results in statistics. The first half of this treatise was concerned with probability methods and problems, the second half with statistical methods and applications. Laplace's proofs are not always rigorous according to the standards of a later day, and his perspective slides back and forth between the Bayesian and non-Bayesian views with an ease that makes some of his investigations difficult to follow, but his conclusions remain basically sound even in those few situations where his analysis goes astray. In 1819, he published a popular account of his work on probability. This book bears the same relation to the Théorie des probabilités that the Système du monde does to the Mécanique céleste. In its emphasis on the analytical importance of probabilistic problems, especially in the context of the "approximation of formula functions of large numbers," Laplace's work goes beyond the contemporary view which almost exclusively considered aspects of practical applicability. Laplace's Théorie analytique remained the most influential book of mathematical probability theory to the end of the 19th century. The general relevance for statistics of Laplacian error theory was appreciated only by the end of the 19th century. However, it influenced the further development of a largely analytically oriented probability theory.

Inductive probability

In his Essai philosophique sur les probabilités (1814), Laplace set out a mathematical system of inductive reasoning based on probability, which we would today recognise as Bayesian. He begins the text with a series of principles of probability, the first seven being:

1. Probability is the ratio of the "favored events" to the total possible events.
2. The first principle assumes equal probabilities for all events. When this is not true, we must first determine the probabilities of each event. Then, the probability is the sum of the probabilities of all possible favoured events.
3. For independent events, the probability of the occurrence of all is the probability of each multiplied together.
4. When two events A and B depend on each other, the probability of compound event is the probability of A multiplied by the probability that, given A, B will occur.
5. The probability that A will occur, given that B has occurred, is the probability of A and B occurring divided by the probability of B.
6. Three corollaries are given for the sixth principle, which amount to Bayesian rule. Where event A_i ∈ {A₁, A₂, ... A_n} exhausts the list of possible causes for event B, Pr(B) = Pr(A₁, A₂, ..., A_n). Then ${\displaystyle Pr(A_i\mid B)=Pr(A_i)\frac{Pr(B\mid A_i)}{\sum _{j}Pr(A_j)Pr(B\mid A_j)}.}$
7. The probability of a future event C is the sum of the products of the probability of each causes B_i drawn from the event observed A, by the probability that, this cause existing, the future event will occur. Symbolically, ${\displaystyle Pr(C|A)=\sum _{i}Pr(C|B_i)Pr(B_i|A).}$

One well-known formula arising from his system is the rule of succession, given as principle seven. Suppose that some trial has only two possible outcomes, labelled "success" and "failure". Under the assumption that little or nothing is known a priori about the relative plausibilities of the outcomes, Laplace derived a formula for the probability that the next trial will be a success.

${\displaystyle Pr(\text{next outcome is success})=\frac{s+1}{n+2}}$

where s is the number of previously observed successes and n is the total number of observed trials. It is still used as an estimator for the probability of an event if we know the event space, but have only a small number of samples.

The rule of succession has been subject to much criticism, partly due to the example which Laplace chose to illustrate it. He calculated that the probability that the sun will rise tomorrow, given that it has never failed to in the past, was

${\displaystyle Pr(\text{sun will rise tomorrow})=\frac{d+1}{d+2}}$

where d is the number of times the sun has risen in the past. This result has been derided as absurd, and some authors have concluded that all applications of the Rule of Succession are absurd by extension. However, Laplace was fully aware of the absurdity of the result; immediately following the example, he wrote, "But this number [i.e., the probability that the sun will rise tomorrow] is far greater for him who, seeing in the totality of phenomena the principle regulating the days and seasons, realizes that nothing at the present moment can arrest the course of it."

Probability-generating function

The method of estimating the ratio of the number of favourable cases to the whole number of possible cases had been previously indicated by Laplace in a paper written in 1779. It consists of treating the successive values of any function as the coefficients in the expansion of another function, with reference to a different variable. The latter is therefore called the probability-generating function of the former. Laplace then shows how, by means of interpolation, these coefficients may be determined from the generating function. Next he attacks the converse problem, and from the coefficients he finds the generating function; this is effected by the solution of a finite difference equation.

Least squares and central limit theorem

The fourth chapter of this treatise includes an exposition of the method of least squares, a remarkable testimony to Laplace's command over the processes of analysis. In 1805 Legendre had published the method of least squares, making no attempt to tie it to the theory of probability. In 1809 Gauss had derived the normal distribution from the principle that the arithmetic mean of observations gives the most probable value for the quantity measured; then, turning this argument back upon itself, he showed that, if the errors of observation are normally distributed, the least squares estimates give the most probable values for the coefficients in regression situations. These two works seem to have spurred Laplace to complete work toward a treatise on probability he had contemplated as early as 1783.

In two important papers in 1810 and 1811, Laplace first developed the characteristic function as a tool for large-sample theory and proved the first general central limit theorem. Then in a supplement to his 1810 paper written after he had seen Gauss's work, he showed that the central limit theorem provided a Bayesian justification for least squares: if one were combining observations, each one of which was itself the mean of a large number of independent observations, then the least squares estimates would not only maximise the likelihood function, considered as a posterior distribution, but also minimise the expected posterior error, all this without any assumption as to the error distribution or a circular appeal to the principle of the arithmetic mean. In 1811 Laplace took a different non-Bayesian tack. Considering a linear regression problem, he restricted his attention to linear unbiased estimators of the linear coefficients. After showing that members of this class were approximately normally distributed if the number of observations was large, he argued that least squares provided the "best" linear estimators. Here it is "best" in the sense that it minimised the asymptotic variance and thus both minimised the expected absolute value of the error, and maximised the probability that the estimate would lie in any symmetric interval about the unknown coefficient, no matter what the error distribution. His derivation included the joint limiting distribution of the least squares estimators of two parameters.

Laplace's demon

Main article: Laplace's demon

In 1814, Laplace published what may have been the first scientific articulation of causal determinism:

We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be the present to it.

— Pierre Simon Laplace, A Philosophical Essay on Probabilities

This intellect is often referred to as Laplace's demon (in the same vein as Maxwell's demon) and sometimes Laplace's Superman (after Hans Reichenbach). Laplace, himself, did not use the word "demon", which was a later embellishment. As translated into English above, he simply referred to: "Une intelligence ... Rien ne serait incertain pour elle, et l'avenir comme le passé, serait présent à ses yeux."

Even though Laplace is generally credited with having first formulated the concept of causal determinism, in a philosophical context the idea was actually widespread at the time, and can be found as early as 1756 in Maupertuis' 'Sur la Divination'. As well, Jesuit scientist Boscovich first proposed a version of scientific determinism very similar to Laplace's in his 1758 book Theoria philosophiae naturalis.

Laplace transforms

Main article: Laplace transform § History

As early as 1744, Euler, followed by Lagrange, had started looking for solutions of differential equations in the form:

${\displaystyle z=\int X(x)e^{ax}dx\text{ and }z=\int X(x)x^{a}dx.}$

The Laplace transform has the form:

${\displaystyle F(s)=\int f(t)e^{-st}dt}$

This integral operator transforms a function of time (${\displaystyle t}$) into a function of a new variable (${\displaystyle s}$).

Other discoveries and accomplishments

Mathematics

Among the other discoveries of Laplace in pure and applied mathematics are:

• Discussion, contemporaneously with Alexandre-Théophile Vandermonde, of the general theory of determinants, (1772);
• Proof that every equation of an odd degree must have at least one real quadratic factor
• Laplace's method for approximating integrals
• Solution of the linear partial differential equation of the second order;
• He was the first to consider the difficult problems involved in equations of mixed differences, and to prove that the solution of an equation in finite differences of the first degree and the second order might always be obtained in the form of a continued fraction;
• In his theory of probabilities:
  • de Moivre–Laplace theorem that approximates binomial distribution with a normal distribution
  • Evaluation of several common definite integrals;
  • General proof of the Lagrange reversion theorem.

Surface tension

Main article: Young–Laplace equation § History

Laplace built upon the qualitative work of Thomas Young to develop the theory of capillary action and the Young–Laplace equation.

Speed of sound

Laplace in 1816 was the first to point out that the speed of sound in air depends on the heat capacity ratio. Newton's original theory gave too low a value, because it does not take account of the adiabatic compression of the air which results in a local rise in temperature and pressure. Laplace's investigations in practical physics were confined to those carried on by him jointly with Lavoisier in the years 1782 to 1784 on the specific heat of various bodies.

Politics

Minister of the Interior

In his early years, Laplace was careful never to become involved in politics, or indeed in life outside the Académie des sciences. He prudently withdrew from Paris during the most violent part of the Revolution.

In November 1799, immediately after seizing power in the coup of 18 Brumaire, Napoleon appointed Laplace to the post of Minister of the Interior. The appointment, however, lasted only six weeks, after which Lucien Bonaparte, Napoleon's brother, was given the post. Evidently, once Napoleon's grip on power was secure, there was no need for a prestigious but inexperienced scientist in the government. Napoleon later (in his Mémoires de Sainte Hélène) wrote of Laplace's dismissal as follows:

Geometrician of the first rank, Laplace was not long in showing himself a worse than average administrator; from his first actions in office we recognized our mistake. Laplace did not consider any question from the right angle: he sought subtleties everywhere, conceived only problems, and finally carried the spirit of "infinitesimals" into the administration.

Grattan-Guinness, however, describes these remarks as "tendentious", since there seems to be no doubt that Laplace "was only appointed as a short-term figurehead, a place-holder while Napoleon consolidated power".

From Bonaparte to the Bourbons

Laplace

Although Laplace was removed from office, it was desirable to retain his allegiance. He was accordingly raised to the senate, and to the third volume of the Mécanique céleste he prefixed a note that of all the truths therein contained the most precious to the author was the declaration he thus made of his devotion towards the peacemaker of Europe. In copies sold after the Bourbon Restoration this was struck out. (Pearson points out that the censor would not have allowed it anyway.) In 1814 it was evident that the empire was falling; Laplace hastened to tender his services to the Bourbons, and in 1817 during the Restoration he was rewarded with the title of marquis.

According to Rouse Ball, the contempt that his more honest colleagues felt for his conduct in the matter may be read in the pages of Paul Louis Courier. His knowledge was useful on the numerous scientific commissions on which he served, and, says Rouse Ball, probably accounts for the manner in which his political insincerity was overlooked.

Roger Hahn in his 2005 biography disputes this portrayal of Laplace as an opportunist and turncoat, pointing out that, like many in France, he had followed the debacle of Napoleon's Russian campaign with serious misgivings. The Laplaces, whose only daughter Sophie had died in childbirth in September 1813, were in fear for the safety of their son Émile, who was on the eastern front with the emperor. Napoleon had originally come to power promising stability, but it was clear that he had overextended himself, putting the nation at peril. It was at this point that Laplace's loyalty began to weaken. Although he still had easy access to Napoleon, his personal relations with the emperor cooled considerably. As a grieving father, he was particularly cut to the quick by Napoleon's insensitivity in an exchange related by Jean-Antoine Chaptal: "On his return from the rout in Leipzig, he [Napoleon] accosted Mr Laplace: 'Oh! I see that you have grown thin—Sire, I have lost my daughter—Oh! that's not a reason for losing weight. You are a mathematician; put this event in an equation, and you will find that it adds up to zero.'"

Political philosophy

In the second edition (1814) of the Essai philosophique, Laplace added some revealing comments on politics and governance. Since it is, he says, "the practice of the eternal principles of reason, justice and humanity that produce and preserve societies, there is a great advantage to adhere to these principles, and a great inadvisability to deviate from them". Noting "the depths of misery into which peoples have been cast" when ambitious leaders disregard these principles, Laplace makes a veiled criticism of Napoleon's conduct: "Every time a great power intoxicated by the love of conquest aspires to universal domination, the sense of liberty among the unjustly threatened nations breeds a coalition to which it always succumbs." Laplace argues that "in the midst of the multiple causes that direct and restrain various states, natural limits" operate, within which it is "important for the stability as well as the prosperity of empires to remain". States that transgress these limits cannot avoid being "reverted" to them, "just as is the case when the waters of the seas whose floor has been lifted by violent tempests sink back to their level by the action of gravity".

About the political upheavals he had witnessed, Laplace formulated a set of principles derived from physics to favour evolutionary over revolutionary change:

Let us apply to the political and moral sciences the method founded upon observation and calculation, which has served us so well in the natural sciences. Let us not offer fruitless and often injurious resistance to the inevitable benefits derived from the progress of enlightenment; but let us change our institutions and the usages that we have for a long time adopted only with extreme caution. We know from past experience the drawbacks they can cause, but we are unaware of the extent of ills that change may produce. In the face of this ignorance, the theory of probability instructs us to avoid all change, especially to avoid sudden changes which in the moral as well as the physical world never occur without a considerable loss of vital force.

In these lines, Laplace expressed the views he had arrived at after experiencing the Revolution and the Empire. He believed that the stability of nature, as revealed through scientific findings, provided the model that best helped to preserve the human species. "Such views," Hahn comments, "were also of a piece with his steadfast character."

In the Essai philosophique, Laplace also illustrates the potential of probabilities in political studies by applying the law of large numbers to justify the candidates’ integer-valued ranks used in the Borda method of voting, with which the new members of the Academy of Sciences were elected. Laplace’s verbal argument is so rigorous that it can easily be converted into a formal proof.

Death

Tomb of Pierre-Simon Laplace

Laplace died in Paris on 5 March 1827, which was the same day Alessandro Volta died. His brain was removed by his physician, François Magendie, and kept for many years, eventually being displayed in a roving anatomical museum in Britain. It was reportedly smaller than the average brain. Laplace was buried at Père Lachaise in Paris but in 1888 his remains were moved to Saint Julien de Mailloc in the canton of Orbec and reinterred on the family estate. The tomb is situated on a hill overlooking the village of St Julien de Mailloc, Normandy, France.

Religious opinions

I had no need of that hypothesis

See also: God of the gaps

A frequently cited but potentially apocryphal interaction between Laplace and Napoleon purportedly concerns the existence of God. Although the conversation in question did occur, the exact words Laplace used and his intended meaning are not known. A typical version is provided by Rouse Ball:

Laplace went in state to Napoleon to present a copy of his work, and the following account of the interview is well authenticated, and so characteristic of all the parties concerned that I quote it in full. Someone had told Napoleon that the book contained no mention of the name of God; Napoleon, who was fond of putting embarrassing questions, received it with the remark, 'M. Laplace, they tell me you have written this large book on the system of the universe, and have never even mentioned its Creator.' Laplace, who, though the most supple of politicians, was as stiff as a martyr on every point of his philosophy, drew himself up and answered bluntly, Je n'avais pas besoin de cette hypothèse-là. ("I had no need of that hypothesis.") Napoleon, greatly amused, told this reply to Lagrange, who exclaimed, Ah! c'est une belle hypothèse; ça explique beaucoup de choses. ("Ah, it is a fine hypothesis; it explains many things.")

An earlier report, although without the mention of Laplace's name, is found in Antommarchi's The Last Moments of Napoleon (1825):

Je m'entretenais avec L ..... je le félicitais d'un ouvrage qu'il venait de publier et lui demandais comment le nom de Dieu, qui se reproduisait sans cesse sous la plume de Lagrange, ne s'était pas présenté une seule fois sous la sienne. C'est, me répondit-il, que je n'ai pas eu besoin de cette hypothèse. ("While speaking with L ..... I congratulated him on a work which he had just published and asked him how the name of God, which appeared endlessly in the works of Lagrange, didn't occur even once in his. He replied that he had no need of that hypothesis.")

In 1884, however, the astronomer Hervé Faye affirmed that this account of Laplace's exchange with Napoleon presented a "strangely transformed" (étrangement transformée) or garbled version of what had actually happened. It was not God that Laplace had treated as a hypothesis, but merely his intervention at a determinate point:

In fact Laplace never said that. Here, I believe, is what truly happened. Newton, believing that the secular perturbations which he had sketched out in his theory would in the long run end up destroying the Solar System, says somewhere that God was obliged to intervene from time to time to remedy the evil and somehow keep the system working properly. This, however, was a pure supposition suggested to Newton by an incomplete view of the conditions of the stability of our little world. Science was not yet advanced enough at that time to bring these conditions into full view. But Laplace, who had discovered them by a deep analysis, would have replied to the First Consul that Newton had wrongly invoked the intervention of God to adjust from time to time the machine of the world (la machine du monde) and that he, Laplace, had no need of such an assumption. It was not God, therefore, that Laplace treated as a hypothesis, but his intervention in a certain place.

Laplace's younger colleague, the astronomer François Arago, who gave his eulogy before the French Academy in 1827, told Faye of an attempt by Laplace to keep the garbled version of his interaction with Napoleon out of circulation. Faye writes:

I have it on the authority of M. Arago that Laplace, warned shortly before his death that that anecdote was about to be published in a biographical collection, had requested him [Arago] to demand its deletion by the publisher. It was necessary to either explain or delete it, and the second way was the easiest. But, unfortunately, it was neither deleted nor explained.

The Swiss-American historian of mathematics Florian Cajori appears to have been unaware of Faye's research, but in 1893 he came to a similar conclusion. Stephen Hawking said in 1999, "I don't think that Laplace was claiming that God does not exist. It's just that he doesn't intervene, to break the laws of Science."

The only eyewitness account of Laplace's interaction with Napoleon is from the entry for 8 August 1802 in the diary of the British astronomer Sir William Herschel:

The first Consul then asked a few questions relating to Astronomy and the construction of the heavens to which I made such answers as seemed to give him great satisfaction. He also addressed himself to Mr Laplace on the same subject, and held a considerable argument with him in which he differed from that eminent mathematician. The difference was occasioned by an exclamation of the first Consul, who asked in a tone of exclamation or admiration (when we were speaking of the extent of the sidereal heavens): 'And who is the author of all this!' Mons. De la Place wished to shew that a chain of natural causes would account for the construction and preservation of the wonderful system. This the first Consul rather opposed. Much may be said on the subject; by joining the arguments of both we shall be led to 'Nature and nature's God'.

Since this makes no mention of Laplace's saying, "I had no need of that hypothesis," Daniel Johnson argues that "Laplace never used the words attributed to him." Arago's testimony, however, appears to imply that he did, only not in reference to the existence of God.

Views on God

Raised a Catholic, Laplace appears in adult life to have inclined to deism (presumably his considered position, since it is the only one found in his writings). However, some of his contemporaries thought he was an atheist, while a number of recent scholars have described him as agnostic.

Faye thought that Laplace "did not profess atheism", but Napoleon, on Saint Helena, told General Gaspard Gourgaud, "I often asked Laplace what he thought of God. He owned that he was an atheist." Roger Hahn, in his biography of Laplace, mentions a dinner party at which "the geologist Jean-Étienne Guettard was staggered by Laplace's bold denunciation of the existence of God." It appeared to Guettard that Laplace's atheism "was supported by a thoroughgoing materialism." But the chemist Jean-Baptiste Dumas, who knew Laplace well in the 1820s, wrote that Laplace "provided materialists with their specious arguments, without sharing their convictions."

Hahn states: "Nowhere in his writings, either public or private, does Laplace deny God's existence." Expressions occur in his private letters that appear inconsistent with atheism. On 17 June 1809, for instance, he wrote to his son, "Je prie Dieu qu'il veille sur tes jours. Aie-Le toujours présent à ta pensée, ainsi que ton père et ta mère [I pray that God watches over your days. Let Him be always present to your mind, as also your father and your mother]." Ian S. Glass, quoting Herschel's account of the celebrated exchange with Napoleon, writes that Laplace was "evidently a deist like Herschel".

In Exposition du système du monde, Laplace quotes Newton's assertion that "the wondrous disposition of the Sun, the planets and the comets, can only be the work of an all-powerful and intelligent Being." This, says Laplace, is a "thought in which he [Newton] would be even more confirmed, if he had known what we have shown, namely that the conditions of the arrangement of the planets and their satellites are precisely those which ensure its stability." By showing that the "remarkable" arrangement of the planets could be entirely explained by the laws of motion, Laplace had eliminated the need for the "supreme intelligence" to intervene, as Newton had "made" it do. Laplace cites with approval Leibniz's criticism of Newton's invocation of divine intervention to restore order to the Solar System: "This is to have very narrow ideas about the wisdom and the power of God." He evidently shared Leibniz's astonishment at Newton's belief "that God has made his machine so badly that unless he affects it by some extraordinary means, the watch will very soon cease to go."

In a group of manuscripts, preserved in relative secrecy in a black envelope in the library of the Académie des sciences and published for the first time by Hahn, Laplace mounted a deist critique of Christianity. It is, he writes, the "first and most infallible of principles ... to reject miraculous facts as untrue." As for the doctrine of transubstantiation, it "offends at the same time reason, experience, the testimony of all our senses, the eternal laws of nature, and the sublime ideas that we ought to form of the Supreme Being." It is the sheerest absurdity to suppose that "the sovereign lawgiver of the universe would suspend the laws that he has established, and which he seems to have maintained invariably."

Laplace also ridiculed the use of probability in theology. Even following Pascal's reasoning presented in Pascal's wager, it is not worth making a bet, for the hope of profit – equal to the product of the value of the testimonies (infinitely small) and the value of the happiness they promise (which is significant but finite) – must necessarily be infinitely small.

In old age, Laplace remained curious about the question of God and frequently discussed Christianity with the Swiss astronomer Jean-Frédéric-Théodore Maurice. He told Maurice that "Christianity is quite a beautiful thing" and praised its civilising influence. Maurice thought that the basis of Laplace's beliefs was, little by little, being modified, but that he held fast to his conviction that the invariability of the laws of nature did not permit of supernatural events. After Laplace's death, Poisson told Maurice, "You know that I do not share your [religious] opinions, but my conscience forces me to recount something that will surely please you." When Poisson had complimented Laplace about his "brilliant discoveries", the dying man had fixed him with a pensive look and replied, "Ah! We chase after phantoms [chimères]." These were his last words, interpreted by Maurice as a realisation of the ultimate "vanity" of earthly pursuits. Laplace received the last rites from the curé of the Missions Étrangères (in whose parish he was to be buried) and the curé of Arcueil.

According to his biographer, Roger Hahn, it is "not credible" that Laplace "had a proper Catholic end", and he "remained a skeptic" to the very end of his life. Laplace in his last years has been described as an agnostic.

Excommunication of a comet

In 1470 the humanist scholar Bartolomeo Platina wrote that Pope Callixtus III had asked for prayers for deliverance from the Turks during a 1456 appearance of Halley's Comet. Platina's account does not accord with Church records, which do not mention the comet. Laplace is alleged to have embellished the story by claiming the Pope had "excommunicated" Halley's comet. What Laplace actually said, in Exposition du système du monde (1796), was that the Pope had ordered the comet to be "exorcised" (conjuré). It was Arago, in Des Comètes en général (1832), who first spoke of an excommunication.

Honors

• Correspondent of the Royal Institute of the Netherlands in 1809.
• Foreign Honorary Member of the American Academy of Arts and Sciences in 1822.
• The asteroid 4628 Laplace is named for Laplace.
• A spur of the Montes Jura on the Moon is known as Promontorium Laplace.
• His name is one of the 72 names inscribed on the Eiffel Tower.
• The tentative working name of the European Space Agency Europa Jupiter System Mission is the "Laplace" space probe.
• A train station in the RER B in Arcueil bears his name.
• A street in Verkhnetemernitsky (near Rostov-on-Don, Russia).
• The Institute of Electrical and Electronics Engineers (IEEE) Signal Processing Society's Early Career Technical Achievement Award is named in his honor.

Quotations

I had no need of that hypothesis. ("Je n'avais pas besoin de cette hypothèse-là", allegedly as a reply to Napoleon, who had asked why he hadn't mentioned God in his book on astronomy.)

It is therefore obvious that ... (Frequently used in the Celestial Mechanics when he had proved something and mislaid the proof, or found it clumsy. Notorious as a signal for something true, but hard to prove.)

If we seek a cause wherever we perceive symmetry, it is not that we regard a symmetrical event as less possible than the others, but, since this event ought to be the effect of a regular cause or that of chance, the first of these suppositions is more probable than the second.

The more extraordinary the event, the greater the need of its being supported by strong proofs.

"We are so far from knowing all the agents of nature and their diverse modes of action that it would not be philosophical to deny phenomena solely because they are inexplicable in the actual state of our knowledge. But we ought to examine them with an attention all the more scrupulous as it appears more difficult to admit them."

• This is restated in Theodore Flournoy's work From India to the Planet Mars as the Principle of Laplace or, "The weight of the evidence should be proportioned to the strangeness of the facts."
• Most often repeated as "The weight of evidence for an extraordinary claim must be proportioned to its strangeness." (see also: Sagan standard)

This simplicity of ratios will not appear astonishing if we consider that all the effects of nature are only mathematical results of a small number of immutable laws.

Infinitely varied in her effects, nature is only simple in her causes.

What we know is little, and what we are ignorant of is immense. (Fourier comments: "This was at least the meaning of his last words, which were articulated with difficulty.")

One sees in this essay that the theory of probabilities is basically only common sense reduced to a calculus. It makes one estimate accurately what right-minded people feel by a sort of instinct, often without being able to give a reason for it.