A letter by French philosopher René Descartes, stolen more than 150 years ago, has been located at a small college in the U.S.

Dutch scholar Erik-Jan Bos discovered on a Google search that the 1641 letter was mentioned as part of a description of the Charles Roberts Collection at the Haverford College Library, located just outside Philadelphia.

Bos — who is compiling a book about Descartes' correspondences — contacted the college, which examined the four-page letter and determined it to be authentic.

The letter contains an exchange between the philosopher and his friend Marin Mersenne about Descartes' Meditations on First Philosophy.

Documents sold to British collectors

The letter was one of thousands of documents taken by Count Guglielmo Libri Carucci dalla Sommaja in the19th century. Guglielmo had been the secretary of the committee overseeing manuscripts in French public libraries.

He sold the documents to British collectors and it's assumed that the Descartes letter passed through many hands before it was given to the college in 1902 by the widow of collector Charles Roberts.

Bos, who works at the University of Utrecht, calls the letter "an invaluable addition to our understanding of the life and works of Descartes."

He said it reveals how Descartes was influenced by others in his writings and provides a glimpse into how the philosopher clarified his ideas about God.

Haverford president Stephen Emerson said he contacted the Institut de France, the letter's original home.

"There was really only one possible course of action: do the right thing, and offer to return the letter," Emerson told The Guardian newspaper.