The Theory of Money and the Theory of Value.
The most important point to emerge from Marx's theory of money is the idea that money is a form of value. The difficulty with this idea is that we are more familiar with money itself than with value in other forms. But value does appear in forms other than money. For example, the balance sheet of a capitalist firm estimates the value of goods in process and of fixed capital which has not yet been depreciated, as well as the value of inventories of finished commodities awaiting sale. Each of these aggregations of commodities has a value, usually expressed as the equivalent of a certain amount of money, but it is clear that neither goods in process nor fixed capital is money. Marx views the value of commodities in this sense as analytically prior to money; money can be explained according to Marx only on the basis of an understanding of the value of commodities. .
Marx follows Smith in regarding value as the property of exchangeability of commodities. In a society where exchange is common, products come to have a dual character as use values and as values. They have two powers: first, to satisfy particular human needs and wants; and second, to exchange for other products. This second power can be thought of quantitatively, as an amount of exchangeability or command over other commodities. The classical economists viewed value as a real, though socially determined, entity, with its own laws of conservation and motion. Value in this sense bears the same relation to commodities as mass bears to physical objects. .
It is not surprising that in societies where exchange is widespread value takes on an independent form as money, as an expression of general exchangeability. Value is a central social reality for people; they constantly think and talk about it directly or indirectly; they want some way to transfer it directly among themselves, separate from particular commodities.