As soon as Haverford’s president, Stephen G. Emerson, understood the letter’s history, he contacted the Institut de France (coincidentally on Feb. 11, the anniversary of Descartes’ death in 1650) and offered to return the item. “I was frankly shocked because I didn’t know we had the letter at all,” said Mr. Emerson, who was a philosophy major in college. “But it’s really not ours.”

Scholars have known of the letter’s existence for more than 300 years, but not its contents. Apparently the only person who had really studied it was a Haverford undergraduate who spent a semester writing a paper about the letter in 1979. (Mr. Bos called the paper “a truly fine piece of work.”)

Gabriel de Broglie, chancellor of the Institut de France, an organization that manages thousands of donations and foundations, described the letter as “a wonderful discovery for science.”

Delighted by the college’s offer, Mr. de Broglie awarded Haverford a prize of 15,000 euros (slightly more than $20,000), writing to Mr. Emerson that the offer “honors you and exemplifies the depth of moral values that you instill in your students.”

Image The stolen letter, dated May 27, 1641. Credit... Haverford College

France has recovered only 45 of the 72 stolen Descartes letters, Mr. de Broglie explained. One was offered at an auction in Switzerland in 2006 and 2009. “After I protested vociferously and publicly on both occasions in the name of the Institut, the letter didn’t find a buyer,” Mr. de Broglie wrote, “but it proved impossible for us to raise the very large sum that the seller demanded, and even though it can’t be sold, this 1638 letter remains in private hands.”

The letters were among thousands of documents stolen by Guglielmo Libri, an Italian count and mathematician who served as secretary of the Committee for the General Catalog of Manuscripts in French Public Libraries in the 1840s. After learning that he might be arrested, Libri fled to London in 1848 with a collection of 30,000 books and manuscripts, including those by Descartes, Galileo, Fermat, Leibniz, Copernicus and Kepler and other scientific and mathematical giants.