a ) Wage-Labour as a Social Relation
If one looks at modern society, it is obvious that in order to live, the great majority of people are forced to sell their labour power. All the physical and intellectual capacities existing in human beings, in their very personalities, which must be set in motion to produce useful things, can only be used if they are sold in exchange for wages. Labour power is a commodity like all other goods. The existence of exchange and wage-labour seems normal, inevitable. Yet the introduction of wage-labour required violence and was accompanied by social conflicts.  The separation of the worker from the means of production, which has become a fact of life, accepted as such, was actually the result of a long evolution, and could only be accomplished by force. In England, in the Netherlands, in France, from the 16th century on, economic and political violence expropriated craftsmen and peasants, repressed indigence and vagrancy, imposed wage-labour on the poor. In the 20th century, between 1930 and 1950, Russia had to decree a labour code which included capital punishment in order to organise the passage of millions of peasants to industrial wage-labour in a few decades. Seemingly normal facts : that an individual has nothing but his labour power, that he must sell it to an enterprise to be able to live, that everything is a commodity, that social relations revolve around exchange, are the result of a long and violent process.
By means of its school system and its ideological and political life, contemporary society hides the past and present violence on which this situation rests. It hides both its origin and the mechanism which enables it to function. Everything appears to be the result of a free contract in which the individual, as a seller of labour power, encounters the factory. The existence of the commodity seems to be an obvious and natural phenomenon. Yet it causes periodic major and minor disasters : goods are destroyed to maintain their prices, existing capacities are not used, while elementary needs are not fulfilled. The two pillars of modern society, exchange and wage-labour, are not only the source of periodic and constant disasters, but have also created the conditions which make another society possible. Most importantly, they compel a large section of the present world to revolt against them, and to realise this possibility : communism.
To understand this, it is necessary to situate contemporary society in its historical context, to see where it comes from and where it is going. The links between the members of a society, and the links between all the elements which constitute it ( individuals, tools of production, institutions, ideas ) are transitory. They are the effect of a past evolution, and the cause of a future transformation. The relations uniting these elements are dynamic : their present can only be understood through their past and future.
By definition, all human activity is social. Human life only exists in groups, through numerous forms of association. The reproduction of living conditions is a collective activity from the start : both the reproduction of the human beings themselves and the reproduction of their means of existence.  Indeed, what characterises human society is the fact that it produces and reproduces the material conditions of its existence. Some animals use tools, but only man makes his tools. Between the individual or group and the fulfilment of needs comes the mediation of production, of work, which continually modifies the ways to act in and transform the environment. Other forms of life -- bees, for example -- make their own material conditions, but, at least as far as man can study them, their evolution seems at a standstill. Work, by contrast, is a continually changing appropriation and assimilation of man's environment. The relation of men to "nature" is also a relation among men and depends on their relations of production, just as the ideas they produce, the way they conceive the world, depend on their production relations.
The transformation of activity accompanies the transformation of the social context in which it takes place, i.e., the relations among people. Production relations into which people enter are independent of their will : each generation confronts technical and social conditions left by previous generations. But it can alter them, up to the limits allowed by the level of the material productive forces. What people call "history" does not achieve anything : history is made by people, but only to the extent that given possibilities allow. This is not to say that each important change in productive forces is automatically and immediately accompanied by a corresponding change in production relations. If this were true, there would be no revolutions. The new society bred by the old can only appear and triumph through a revolution, by destroying the entire political and ideological structure which until then allowed the survival of obsolete production relations.
Every production relation is historical, hence transitory. Wage-labour is one type of relation among individuals, between the individual and society, between the individual and the production of his means of existence. It is but one production relation in a whole historical evolution. In spite of the misery and suffering it has brought, it has played a useful role, creating the necessary basis for its own destruction. Wage-labour was once a form of development, but it no longer is; for a long time it has been nothing but a hindrance, even a threat to the very existence of mankind. 
What must be exposed, behind the material objects, the machines, the factories, the labourers who work there every day, the things they produce, is the social relation that regulates them, as well as its necessary and possible evolution.
 E. P. Thompson, The Making of the English Working Class ( Pelican, 1970 ).
 Marx and Engels, The German Ideology, Moscow : Progress Publishers, 1964, Part 1.
 See Engels' review of Capital, in Engels, Selected Writings ( Penguin, 1967 ) pp. 177-184.