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 still bears some resemblance to the objects from which it proceeds. As I have amply shown already, however, we must not think that it is by means of this resemblance that the picture causes our sensory perception of these objects—as if there were yet other eyes within our brain with which we could perceive it. Instead we must hold that it is the movements composing this picture which, acting directly upon our soul in so far as it is united to our body, are ordained by nature to make it have such sensations” (AT VI:130, CSM I:167). This remark shows that Descartes tried to avoid the so-called “homuncular fallacy,” which explains perception by assuming that there is a little man in the head who perceives the output of the sense organs, and obviously leads to an infinite regress.