René Descartes (1596-1650) and Marin Mersenne (1588-1648), two of the greatest 17th century French minds, carried on a regular correspondence. In this lengthy 1640 letter to Mersenne, Descartes ranges widely, discussing a dispute with Pierre Bourdin, a Jesuit who had advanced a number of objections to Descartes’ Meditationes de Prima Philosophia, then circulating in manuscript…