Revisiting the Logical Problem of Evil via Modal Tension

J.L. Mackie’s logical problem of evil is widely considered addressed by Alvin Plantinga’s Free Will Defense (FWD). Plantinga’s defense heavily relies on the logical possibility of libertarian free will. This paper argues that Plantinga’s defense faces a significant metaphysical tension when situated within Classical Theism. By formulating a deductive argument utilizing modal logic and the Principle of Sufficient Reason (PSR), I demonstrate that the core attributes of the Classical Theist God—namely, absolute immutability and perfect rationality—create a strong tendency toward a modal tension. A necessarily static and rational cause appears to entail a deterministic universe. Consequently, libertarian free will is shown to be difficult to reconcile ontologically with this framework, thereby posing a profound challenge to Plantinga’s FWD and revisiting the logical problem of evil.