In classical political economy and especially Karl Marx's critique of political economy, a commodity is any good or service ("products" or "activities") 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 though they may not be produced specifically for the market, or be non-reproducible goods. 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.