Occasionalism is the doctrine that relegates all real causal efficacy exclusively to God. This paper will aim to elucidate in some detail the metaphysical considerations that, together with certain common medieval theological axioms, constitute the philosophical steps leading to this doctrine. First, I will explain how the doctrine of divine conservation implies that we should attribute to divine power causal immediacy in every natural event and that it rules out mere conservationism as a model of the causal relation between God and nature. This leaves concurrentism and occasionalism as the only compatible options. Then I will explain the argument that since no coherent conception of divine concurrence is possible, occasionalism emerges as the only model of the causal relation between God and nature compatible with the doctrine of divine conservation.