Theories differ according to which of the factors shaping price they focus on. Marx and Marxists, for example, focus on the labor needed to produce a commodity. They recognize the other factors, but believe that special insights into the economy and society become available if one uses a labor-focused approach. This approach was first used extensively by Adam Smith and David Ricardo, the founders of economics as a discipline and both avid supporters and even celebrators of capitalism. Marx borrowed (with fullsome praise for them) what they called the labor theory of value. But Marx used it in ways Smith and Ricardo had not done. Marx used it to show that the value added by the worker when he/she worked was always greater, in capitalism, than the value of the wage paid to the worker for his/her work. That difference between the value added by the worker (realized by the employer when the commodity is sold) and the value paid to that worker is, for Marx and Marxists, the source of profit. They call it a "surplus." Capitalism is thus an economic system founded upon the delivery of a surplus by workers to capitalists and that surplus delivery, Marx and Marxists show, has all sorts of consequences for capitalist societies. The production of surpluses by the majority and their receipt and use by a different group of people, indeed a minority, is what Marx and Marxists call exploitation. They find it unjust, they find its economic and social consequences unjust and wasteful of nature and human resources, and they work to change society in the belief that societies can and should do better than capitalism.