More than any other single thinker, William of Ockham (c.1285–1347) is responsible for the widely held modern assumption that religious and secular-political institutions should normally operate independently of one another. Today, when this assumption is questioned in some quarters, Ockham’s acute analysis of the basis and functions of authority in spiritual and temporal affairs is of current as well as historical interest. His point of departure was a tragic collision between two specifically Christian ideals: the Franciscan conception of Christ’s lordship (as lacking material wealth and power) and the ideal of a society guided by the single supreme authority of Christ’s vicar, the Pope. This volume begins with Ockham’s personal account of his engagement in that conflict and continues with essential passages from the major works in which he attempted to resolve it.