An opponent of philosophical rationalists, Hume held that passions rather than reason govern human behaviour, famously proclaiming that "Reason is, and ought only to be the slave of the passions." Hume was also a sentimentalist who held that ethics are based on emotion or sentiment rather than abstract moral principle.
Smith argued against mercantilism and was a major proponent of laissez-faire economic policies. In his first book, "The Theory of Moral Sentiments," Smith proposed the idea of an invisible hand—the tendency of free markets to regulate themselves by means of competition, supply and demand, and self-interest.