commodities are defined as objects which are offered for sale or are "exchanged in a market". In the marketplace, where commodities are sold, "use value" is not helpful in facilitating the sale of commodities. Accordingly, in addition to having use value, commodities must have an "exchange value"—a value that could be expressed in the market. Prior to Marx, many economists debated as to what elements made up exchange value. Adam Smith maintained that exchange value was made up of rent, profit, labour and the costs of wear and tear on the instruments of husbandry. David Ricardo, a follower of Adam Smith, modified Smith's approach on this point by alleging that labour alone is the content of the exchange value of any good or service. While maintaining that all exchange value in commodities was derived directly from the hands of the people that made the commodity, Ricardo noted that only part of the exchange value of the commodity was paid to the worker who made the commodity. The other part of the value of this particular commodity was labour that was not paid to the worker—unpaid labour. This unpaid labour was retained by the owner of the means of production. In capitalist society, the capitalist owns the means of production and therefore the unpaid labour is retained by the capitalist as rent or as profit. The means of production means the site where the commodity is made, the raw products that are used in the production and the instruments or machines that are used for the production of the commodity. However, not all commodities are reproducible nor were all commodities originally intended to be sold in the market. These priced goods are also treated as commodities, e.g. human labour-power, works of art and natural resources ("earth itself is an instrument of labour"), even though 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 labour theory of value. This problem was extensively debated by Adam Smith, David Ricardo and Karl Rodbertus-Jagetzow among others. All three of the above-mentioned economists rejected the theory that labour composed 100% of the exchange value of any commodity. In varying degrees, these economists turned to supply and demand to establish the price of commodities. Marx held that the "price" and the