This chapter discusses the question of the epistemological and ethical justification of an act of belief and of the will's place and role in the production of an assent of faith. It concentrates on three authors: Durand of Saint-Pourçain, Walter Chatton and William of Ockham. I show that Durand and Chatton concur in granting the intellect and its reasoning, theoretical or practical, a prior role in the causal process of the production and justification of an act of belief. Ockham, by contrast, defends a radical voluntarism insofar as he claims that an act of will alone lies at the very root of the causal chain that leads to the production of an act of belief. Current interpretations of Ockham's doxastic voluntarism are also discussed.