Descartes’ short remarks about a small gland in the middle of the brain which is of paramount importance apparently generated a lot of interest. In 1640, Descartes wrote several letters to answer a number of questions that various persons had raised. In these letters, he not only identified the small gland as the conarion or pineal gland (29 January 1640, AT III:19, CSMK 143), but also added some interesting points to the Treatise of man. First, he explained why he regarded it as the principal s