"I have no need of that hypothesis"

According to a story commonly gets told, Laplace presented Napoleon with a copy of his book on the System of the World; Napoleon notes that Laplace did not mention God in it; Laplace replies, "I have no need of that hypothesis."

I have come across several occasions on which people have claimed this story is apocryphal. This is not quite true; it is fictionalized history. The real story, which happened in August of 1802, is given by Sir William Herschel (among other things, the discoverer of Uranus, binary star systems, the first asteroid, and infrared rays):

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'.

[Source: Herschel's diary of his visit to Paris in 1802, as found in C. Lubbock's _The Herschel Chronicle_, p. 310.]

I haven't been able to check this up yet, but I suspect the popular version is due in some form to Bell's _Men of Mathematics_, which is sub-titled 'A Novel', and which is on rare occasions cited as the source of the story. Lubbock's _The Herschel Chronicle_ came out a few years before Bell's book originally did, and so is almost certainly where Bell found the story he adapted for his book.

If you search for the punch-line by search engine you'll find several misattributions, and some of them fairly blatant. Pascal, for instance, despite being unimpressed by natural theology, would never have said. Nor, although it might not be so obvious to those who've never read him, would Voltaire, who criticized the atheist d'Holbach for thinking one could have a system of the world without God. But even when they get the right characters, other mistakes get made. It is fictionalized, but it is also historical in origin.

As I said, I've found a few, even in history of science, who have simply dismissed the story as made up (apparently they've never read up on Sir William Herschel); so I thought I'd post the information here so if you ever use the story you can cite it properly, or if you run across people who treat the story as pure fiction you can correct them and prevent the misinformation from spreading. It's also interesting, simply for how stories get changed through time, that in Herschel's version it's actually the theist that gets the upper hand, since Herschel as a devout theist gets to draw the moral of the story, and Laplace doesn't get the witty agnostic line.