Against the philosophical rationalists, Hume argues that the passions, rather than reason, govern human behaviour. He introduces the famous problem of induction, arguing that inductive reasoning and our beliefs regarding cause and effect cannot be justified by reason; instead, our faith in induction and causation is the result of mental habit and custom. Hume defends a sentimentalist account of morality, arguing that ethics is based on sentiment and the passions rather than reason, and famously declaring that "reason is, and ought only to be the slave to the passions".