The previously unknown letter was found by Bos through Google. "I regularly browse online. A month ago, I was on one of my little forays when I stumbled upon something I hadn't seen before." The document Bos found was a summary of autographs (handwritten, signed texts) that mentioned the letter. The collection the summary referred to is the property of a Quaker-run college in Haverford, Pennsylvania. "They didn't know this letter had never been published before," Bos said. The newly discovered letter is only the third by Descartes found in the last 25 years.

An excerpt from Descartes' letter

I have met Mr. Picot here. It is clear that he is a sensible man and I owe him a lot. A nobleman from Touraine is in his company, and he has sent me the warmest regards on behalf of father Bourdin, of whom he is a student. He has spoken about Mr. Petit in such terms that I feel obliged to moderate my criticism of him, as you will see in the reader's introduction. I am sending it to you with the friendly request to have it printed at the beginning of the book, after the dedication to the lords of the Sorbonne.

Nor the fourth part of the discours de la méthode, nor the short introduction following it, nor the introduction preceding the objections raised by the theologians are to be printed, only the synopsis.

Finally, I can assure you that there is nothing about Mr. Gassendi's objections that I have trouble with. The only thing I should pay heed to is style – since he took such great care to express himself so eloquently, I should try to respond in kind.

I am

Your most obliged and caring servant

Des Cartes

In May of 1641, Descartes had been living in the Dutch Republic for 13 years. He resided in Endegeest castle near Leiden, and had just completed one of his greatest works: Meditationes de prima philosophia (Meditations on first philosophy). In this book he demonstrates the existence of God and argues against transubstantiation, the rendering of wine into Christ's blood and bread into his body Catholics commonly believe occurs during Holy Communion.

The monk Mersenne assisted in the printing of Descartes work in Paris and corresponded with the philosopher. Descartes was looking to include commentary by prominent theologians in his Meditations. He hoped exposure to others' ideas would improve his own, and, as a devout Catholic, was also looking to avoid censorship by the Church. This would prove unnecessary until 13 years after Descartes death. It wouldn't be until 1663 that the pope would put Descartes' works on the Index of Prohibited Books.

Mersenne was charged with collecting the arguments countering Descartes, and the newly discovered letter deals with these matters. "Descartes had just received objections raised by another philosopher, a priest called Pierre Gassendi (1582-1655)," Verbeek said. "Descartes was always very negative about Gassendi. He said he was no good and wrote nothing but rubbish. In this letter however, Descartes seems to be quite satisfied with his objections. He finds them provocative and well written."

Descartes mentions two visitors from France at the bottom of the letter. One was Claude Picot, who would later translate Descartes' Principia Philosophiae ('Principles of philosophy', 1644). The other, a man from Touraine, convinced him to moderate his scathing criticism of another Pierre Petit, a doctor and humanist Descartes had issues with.

After Mersenne's death in 1648, the letter became the property of the French mathematician Gilles de Roberval. When he died in 1675, the French Academy of the Sciences watched over the document for more than a century, until it was stolen by count Guglielmo Libri (1803-1869), a notorious kleptomaniac.

"Libri started out as a legitimate collector and published some of Descartes' letters. When was appointed secretary of a committee charged with inventorying historic documents however, he proved to be unable to resist temptation and stole thousands of autographs. He later sold some of these at auctions in France, Italy and England," Verbeek explained.

An American collector, Charles Roberts (1846-1902), purchased the letter at an auction in the UK. After his death, he bequeathed his collection to his fellow Quakers at Haverford College.

When the college learnt the letter had been stolen it decided to return it to it former owners. It has since transferred the letter to the French Institute, of which the Academy of the Sciences is a part.

The Institute has granted Haverford College a Grand Prize for "good custodianship" in reward.