Thomas Robert Malthus
Physiocratic economists categorized production into productive labour and unproductive labour. Adam Smith expanded this thought by arguing that any economic activities directly related on material products (goods) were productive, and those activities which involved non-material production (services) were unproductive. This emphasis on material production was adapted by David Ricardo, Thomas Robert Malthus and John Stuart Mill, and influenced later Marxian economics. Other, mainly Italian, 18th-century economists maintained that all desired goods and services were productive.