The value of the commodity depends on the capital involved in its production. This capital has two parts constant capital and variable capital. Constant capita relates to means of production like raw material, machinery, tools etc. used for commodity production. The variable capital refers to the wages paid to the worker.
It is the Value of what the labourer sells (his labour power). Surplus “value is the difference between the value produced by the worker and what he gets in exchange for this value of his labour. This is called variable capital because it varies from beginning to the end. It begins as value of the labour power and ends as the value produced by that labour power in the form of a commodity. Labour power has thus a unique quality of its ability to create value.
Marx argued that the capitalist appropriates part of the labour of the worker for which he (the worker) does not get paid. Thus, surplus value is unpaid labour of the labourer. It can be variously measured in terms of time as well as in terms of money. Suppose a worker works for ten hours in producing a commodity. He may get paid for only what is equivalent to his eight hours labour.
Thus, his two hours labour has been appropriated by the capitalist: Marx also argued that gradually the proportion of surplus value becomes more and more. In the example cited above the worker was not paid for his two hours labour out of ten hours that he had spent in producing a commodity because he was paid only for his eight hours labour.
By and by, the proportion of unpaid labour will increase from two to three, four or five hours. Finally, a stage comes when the worker gets paid only the minimum that is necessary for his survival. (His survival does not mean only his personal survival but also the survival of his family so that when this worker is not able to work (due to old age or death or illness) his children may take his place).
As pointed out above, the working class consists of those who own nothing but their own labour power which they are forced to sell in order to live. According to Marx, the history of capitalist production is a history of struggles by the capitalist to increase his surplus value and resistance by the workers against this increase.
There is a difference in the way in which surplus value was created in the slave society and under feudalism and the way it is created in the capitalist society. In the former the slave or the serf who created surplus value was tied to his master or the feudal lord but in capitalism there is a ‘free contract’ into which the worker ‘voluntarily’ enters with the capitalist.
Of course, this freedom is a myth because the worker has no option but to sell his labour power. He must enter into contract with some capitalist. The only option that he has to choose the capitalist to whom he wants to sell his labour power. Thus this freedom is freedom to choose his exploiter. The slave and the serf did not have this freedom.