Siger of Brabant on Determinism: A Reassessment of De necessitate et contingentia causarum

In this paper I discuss Siger of Brabant’s anti-deterministic argument as developed in De necessitate et contingentia causarum. First, I offer an in-depth reconstruction of how Siger justifi es the contingency of effects in nature: the contingent status of an effect depends only on (the contingent status of) its proximate cause, and not on the First Cause. According to Siger, the First Cause, which is understood as a necessary cause, only determines the necessity of its immediate effect. I, then show that such an account runs into an impasse: it is, in fact, impossible to justify how there can be contingent (i.e., impedible) secondary causes in the chain of causes, once it has been assumed that the First Cause is a necessary (i.e., nonimpedible) cause which immediately produces a necessary effect.