Behind the third ventricle of the human brain a miniscule pedunculate bud, close to the optic thalamus, that is, to the two beds of optic nerves, a gland soft in substance yet containing gritty particles. Function: unknown. Because of its pine-cone shape it is called the conarium or pineal body, even though the recent photographs of it by Nilsson and Lindberg show it to be morphologically reminiscent of nothing so much as the plucked tail of a gamebird, which Simon Dedalus refers to as ‘the pope's nose’. Today it is presumed to be an endocrine gland of some sort, even though there is no doubt that morphogenetically in all vertebrates it is a vestigial unpaired eye. As fossil evidence indicates—and we still find it almost fully developed in some extant amphibians—ancestral vertebrates possessed in addition to the paired bilateral eyes a solitary dorsal eye opening at the top of the skull to the sky. This singular evagination of the brain—something betwixt a visual organ and a gland—seems to hold a special fascination for philosophers. Here we shall consider two of them: René Descartes (1596–1650), the father, as we say, of modern philosophy; and Georges Bataille (1897–1962), the father, as many say, of post-modern philosophy.