Ren Descartes's Regulae ad directionem ingenii ('Rules for the Direction of the Understanding') is his earliest surviving philosophical treatise, and in many respects his most puzzling text. It is a profoundly original work with few intellectual precursors, and offers the fullest account anywhere in Descartes's work of his theory of method. Yet Descartes left it unfinished, and unpublished, at his death in 1650. The versions currently known to modern...