This essay proposes that René Descartes, although marginalized in the extant critical tradition, plays a pivotal role in the late works of James Joyce. The overt absence of Descartes from Ulysses in particular is here viewed as a deliberate occlusion of Descartes's central or structural importance to that work. While it is difficult to make definitive claims in the current absence of genetic support, a careful consideration of Joyce's likely Cartesian source materials suggests that the life and philosophy of René Descartes might have provided Joyce with an underlying structural framework comparable to that derived from Homer's Odyssey.