Gabriel de Broglie, chancellor of the prominent academic society, proclaimed himself surprised and delighted by the news.
"C'est une grande joie!" ("It is a great joy"), de Broglie said by telephone.
The full contents of the four-page letter - by the philosopher best known for the phrase Cogito, ergo sum ("I think, therefore I am") - are to be published this year.
Bos, a research fellow at Utrecht University in the Netherlands, said the letter offered an invaluable view of Descartes' mind at work - describing how he decided to exclude certain material from his famous work of 1641, Meditationes de prima philosophia.
The find had a special resonance for Haverford president Stephen Emerson, who read Descartes as a philosophy major when he attended the school in the 1970s.
"I remember reading the 'Meditations' as if it were yesterday," he said. "It's the beginning of modern Western philosophy."
The letter is one of 12,000 in a rich trove of autographed documents collected by Haverford alumnus Charles Roberts, who graduated in 1864. They include letters and other paperwork signed by a wide range of literary figures, politicians and other notables, from Abraham Lincoln to Queen Isabella of Spain.
Roberts is believed to have purchased the letter from a dealer, and school officials said there was no indication he knew it was stolen.
The path to that discovery began last month in the Netherlands, when Bos conducted one of his periodic Google searches for Descartes documents. Haverford had recently published a list of the Roberts letters online, and the Descartes missive popped up on Bos' screen.
He immediately fired off an e-mail to John Anderies, head of special collections at the college. Anderies simply scanned the letter and e-mailed it to Bos.
"It was like a Dan Brown experience, but the real stuff," said Bos, referring to the author of The Da Vinci Code. "It was a thrill to see this letter appear on my desktop screen."
The date written on the document, May 27, 1641, matched the date for a letter that was referenced in inventories of Descartes' correspondence. But modern scholars had no idea of the contents of that letter, as it was one of many stolen in the 1800s by a corrupt library official named Guglielmo Libri.
French scholars agreed with Bos and his supervisor, professor Theo Verbeek, that the Haverford letter was indeed the one on the list with the same date. Meanwhile, Haverford's Anderies notified Emerson, the college president, who said he did not hesitate to offer the letter to the French.
"We realized the most important thing was to return it to its proper owner," Emerson said.
Increasingly, museums have come under pressure to return art and other artifacts that have turned out to be stolen; unlike Haverford, some have declined to give them back.
Libri, the long-ago library official, stole 72 Descartes letters from the institute, according to de Broglie, its chancellor. Libri fled to London and was sentenced in absentia in 1850 for those and other thefts. More than half of the letters have been recovered.
At least some of the thefts occurred while Libri was serving as inspector general in charge of French libraries, de Broglie said. Fittingly, his last name means "books" in Italian. Or perhaps not so fittingly, given his questionable stewardship of the assets under his purview.
"It is very ironic," de Broglie said. "It is a very curious story."
Contact staff writer Tom Avril
at 215-854-2430 or firstname.lastname@example.org.