In classical political economy and especially Karl Marx's critique of political economy, a commodity is any good or service produced by human labour and offered as a product for general sale on the market. Some other priced goods are also treated as commodities, e.g. human labor-power, works of art and natural resources, even although they may not be produced specifically for the market, or be non-reproducible goods.Marx's analysis of the commodity is intended to help solve the problem of what establishes the economic value of goods, using the labor theory of value. This problem was extensively debated by Adam Smith, David Ricardo and Karl Rodbertus-Jagetzow among others. Value and price are not equivalent terms in economics, and theorising the specific relationship of value to market price has been a challenge for both liberal and Marxist economists.