The classical political economists, such as Adam Smith and David Ricardo raised the economic question of which kinds of labour contributed to increasing society's wealth, as against activities which do not increase wealth. In the Introduction to The Wealth of Nations, Smith spoke of the "annual labour" and "the necessaries and conveniences" a nation "annually consumes" before explaining that one of the two steps to increase wealth is reducing the amount of "unproductive labour". "Annual" and "annually" refer to a cyclical reproduction process; "unproductive labour" are commodities and services which are not inputs to the next economic circle and are therefore lost to economic growth. In contrast, theories with no such time horizon tend to understand Smith's unproductive labour as referring