Some philosophers think physical explanations stand on their own: what happens, happens because things have the properties they do. Others think that any such explanation is incomplete: what happens in the physical world must be partly due to the laws of nature. Walter Ott examines the debate between these views from Descartes to Hume, and argues that the competing models of causation in the period grow out of the scholastic notion of power. On this Aristotelian view, the connection between cause and effect is logically necessary. Causes are ‘intrinsically directed’ at what they produce. But when the Aristotelian view is faced with the challenge of mechanism, the core notion of a power splits into two distinct models, each of which persists throughout the early modern period. It is only when seen in this light that the key arguments of the period can reveal their true virtues and flaws.