The first remarks about the pineal gland which Descartes published are to be found in his Dioptrics (1637). The fifth discourse of this book contains the thesis that “a certain small gland in the middle of the ventricles” is the seat of the sensus communis, the general faculty of sense (AT VI:129, not in CSM I). In the sixth discourse, we find the following interesting observation on visual perception: “Now, when this picture [originating in the eyes] thus passes to the inside of our head, it st