Descartes’ thesis that “the pineal gland is the seat of the sensus communis” was soon defended by others. The medical student Jean Cousin defended it in Paris in January 1641 (Cousin 1641) and the professor of theoretical medicine Regius defended it in Utrecht in June 1641 (Regius 1641, third disputation). Mersenne described the reaction of Cousin’s audience in a letter to Descartes, but this letter never reached its destination and is now lost (Lokhorst and Kaitaro 2001).