c ) Commodities
Economic and social progress improves the efficiency of human organisation and its capacity to associate the components of the labour process -- first of all labour power. Then appears the difference ( and the opposition ) between workers and non-workers, between those who organise work and those who work. The first towns and great irrigation projects are born out of this increase of productive efficiency. Commerce appears as a special activity : now there are men who do not make a living by producing, but by mediating between the various activities of the separate units of production. A large proportion of goods is nothing but commodities. To be used, to put into practice their use value, their ability to fulfil a need, they must be bought, they must fulfil their exchange value. Otherwise, although they exist as material and concrete objects, they do not exist from the point of view of society. One has no right to use them. This fact proves that the commodity is not just a thing, but first and foremost a social relation ruled by a definite logic, the logic of exchange, and not of the fulfilment of needs. Use value is now just the support of value. Production becomes a sphere distinct from consumption; work becomes a sphere distinct from non-work. Ownership is the legal framework of the separation between activities, between men, between units of production. The slave is a commodity for his owner, who buys a man to make him work.
The existence of a mediation on the level of the organisation of production ( exchange ) is accompanied by the existence of a mediation on the level of the organisation of people : the State is indispensable as a force gathering the elements of society, in the interests of the ruling class. Unification is made necessary by the destruction of the coherence of the primitive community. Society is forced to maintain its cohesion by creating an institution which is nourished by it.
Exchange becomes visible and concrete with the birth of money.  The abstraction, value, is materialised in money, becomes a commodity, and shows its tendency to become independent, to detach itself from what it comes from and represents : use values, real goods. Compared to simple exchange : x quantity of product A against y quantity of product B, money permits a universalisation, where anything can be obtained for a quantity of abstract labour time crystallised in money. Money is labour-time abstracted from labour and solidified in a durable, measurable, transportable form. Money is the visible, even tangible, manifestation of the common element in all commodities -- not two or several commodities, but all possible commodities. Money allows its owner to command the work of others, at any time and anywhere in the world. With money it is possible to escape from the limits of time and space. If primitive communities were so cut off from each other that they were often unable to wage wars, exchange, which first appeared on their fringes, destroyed them. In the most advanced regions of the world, men organised mercantile and warlike States, and commerce and violence began to socialise the world. A tendency towards a universal economy is at work around some great centres from ancient times to the Middle Ages, but it fails to reach its aim. The retreats of the empires, and their destruction, illustrate this succession of failures. Only capitalism creates, from the 16th century on, but mainly in the 19th and 20th centuries, the necessary basis for a durable universal economy.
 Marx, Capital, Vol. 1, Chapters 2 and 3.