William of Ockham, “the Invincible Doctor,” “the More than Subtle Doctor,” is a giant in the history of thought. In the later middle ages only Thomas Aquinas and Duns Scotus are of comparable stature. Ockham is, however, a highly controversial giant. By some accounts, his early work in theology and philosophy shattered an admirable synthesis of biblical faith and Greek reason achieved, preeminently by Aquinas, in a preceding golden age of scholasticism. In another view, this same work of Ockham's is a “harvest,” not a devastation, of earlier Christian reflection. Ockham thus joins Scotus as a leading figure in the fourteenth-century golden age of Oxford scholasticism. These contrary assessments agree in granting particular significance to Ockham's nominalism and his emphasis on divine omnipotence, but they disagree as to what that significance is. To critics who find Ockham destructive of the Thomistic synthesis, his frequent appeal to the principle that “God can bring about whatever it does not involve a contradiction for God to bring about” seems to menace God's rationality and the intelligibility of the universe. If everything is utterly contingent on God's will, what scope is there for reason, God's or our own? Yet, seen from another angle, the same emphasis on divine power draws a necessary line between subjects which human reason can fruitfully address (the universe God has actually chosen to create) and subjects on which philosophical speculation is largely vain (the divine nature and the things God might have willed but has not).