“Seeing Descartes’ handwriting appear on my screen took my breath away.” Descartes, the author of “Cogito, ergo sum” “I think, therefore I am” spent two decades living in Holland.

A long-lost letter by René Descartes has come to light at Haverford College, where it had lain buried in the archives for more than a century, and the discovery could revolutionize our view of ...