The law of value (German: Wertgesetz) is a central concept in Karl Marx's critique of political economy, first expounded in his polemic The Poverty of Philosophy (1847) against Pierre-Joseph Proudhon, with reference to David Ricardo's economics. Most generally, it refers to a regulative principle of the economic exchange of the products of human work: the relative exchange-values of those products in trade, usually expressed by money-prices, are proportional to the average amounts of human labor-time which are currently socially necessary to produce them.