This chapter discusses the relation between causation and moral responsibility. We generally hold adult human beings morally responsible for their actions, yet those action are also events in the natural world, enmeshed in causal chains that extend backwards in time long before the agent’s birth. If the causes in those chains necessitate their effects, it would appear that we must either give up the view that humans are morally responsible for their actions, or embrace the paradoxical view that humans are morally responsible for actions necessitated by events over which they have no control. Tuozzo argues that Aristotle’s causal theory avoids this dilemma by recognising two distinct types of causal chain or nexus. In one of these, the links between cause and effect are indeed necessary, from beginning to end. But chains of this sort are necessarily finite, with a definite beginning and end. Each of these necessary, finite causal chains are also enmeshed in a different sort of causal nexus, one that does extend indefinitely into the past. But this sort of indefinite causal chain is possible only because it contains links that are not necessitated. This enables Aristotle to account for moral responsibility by locating the necessitating cause of a human action in the agent herself. Nonetheless, Tuozzo concludes, Aristotle’s theory does have the paradoxical implication that, although the state of the world at a given time does not necessitate all subsequent events, a complete description of it would, in principle, allow all subsequent events – including human actions – to be predicted.