Derived from writings of Adam Smith and David Ricardo. Based on liberal assumption that individuals , as rationally self-interested creatures (utility maximizers) are the key economic actors (act to achieve the greatest pleasure over pain). Idea that an unregulated market economy tends towards long-run equilibrium, in that the price mechanism (the invisible hand) brings supply and demand into line with one another. This implies a policy of laissez-fair. Economic change via the market is therefore a positive-sum game, in that greater efficiency produces economic growth and benefits everyone.