The "inquisition" (Mihnah) unleashed by the seventh Abbasid caliph, 'Abdallah al-Ma'mun (r. 813-833), has long attracted the attention of modern scholars of the intellectual, political, and religious history of the early Abbasid era. Because this event, which began in 820 and stretched through the reigns of two of al-Ma'mun's successors, appears at a convergence of prominent currents in systematic theology, rationalist thought, theocratic politics, and nascent trends in Shiism and Sunnism, historians have seen it as the key to a wide array of puzzles and problems in early Islamic history. In...