Chapter 1 explains the doctrine of occasionalism. Section 1.1 unpacks the occasionalist claim that God is the only efficient cause, by explaining the concept of an efficient cause, as it was typically understood in medieval and early modern texts. Section 1.2 contrasts occasionalism with a theory of divine providence developed by Thomas Aquinas, which says that God “concurs” with the actions of created substances. Section 1.3 clarifies the difference between occasionalism and the Thomistic theory of divine concurrence using the notion of a causal power: According to this analysis, occasionalism entails that created substances do not have intrinsic active causal powers. Malebranche expresses this claim by saying that created beings are “occasional causes” that merely “give occasion” to God’s actions. However, section 1.4 argues that there is also a Scholastic tradition that uses terms such as “occasion” and “occasional cause” to refer to a type of true efficient cause.