  For most people today it is hard to take divine illumination seriously, hard to view it as anything other than a quaint relic. A first step toward developing a proper perspective on the theory is to see it in its broader context, not as peculiarly Christian or medieval, but as an assumption shared by most premodern philosophers. A second step in the same direction is to identify and to take seriously the philosophical problem that drives illumination theory. In large part, the theory has been invoked to explain rational insight — that is, a priori knowledge. For much of the modern era, philosophers have been preoccupied with empirical knowledge and have not had much interest in this topic. (Recently, this situation has changed notably; for foundational recent treatments, see Bealer 2000 and Bonjour 1998.) To see how something like divine illumination could have ever seemed at all plausible, one has to see how deeply puzzling the phenomenon of rational insight actually is. One way of seeing this, and of seeing how little we understand rational insight, is to look at cases where something goes wrong. A recent biography of the Nobel-prize winning mathematician John Nash describes his long period of mental illness, during which time he held various odd beliefs such as that extraterrestrials were recruiting him to save the world. How could he believe this, a friend asked during a hospital visit, given his devotion to reason and logic?