 Capital Volume 1 begins with an analysis of the idea of commodity production. A commodity is defined as a useful external object, produced for exchange on a market. Thus, two necessary conditions for commodity production are: the existence of a market, in which exchange can take place; and a social division of labour, in which different people produce different products, without which there would be no motivation for exchange. Marx suggests that commodities have both use-value—a use, in other words—and an exchange-value—initially to be understood as their price. Use value can easily be understood, so Marx says, but he insists that exchange value is a puzzling phenomenon, and relative exchange values need to be explained. Why does a quantity of one commodity exchange for a given quantity of another commodity? His explanation is in terms of the labour input required to produce the commodity, or rather, the socially necessary labour, which is labour exerted at the average level of intensity and productivity for that branch of activity within the economy. Thus the labour theory of value asserts that the value of a commodity is determined by the quantity of socially necessary labour time required to produce it.