Marx argued that the expropriation of surplus value by capitalists is the main source of the oppression and exploitation of the members of the working class. What is surplus value, how is it expropriated, and why is this expropriation exploitative? Capitalists are the owners of the forces of production (machines, soil, buildings, infrastructure, etc.). This exclusive ownership implies the monopolization of the means of labor, and this implies the monopolization of the sources of life.
A monopoly like this doesn’t merely create dependence and therefore precarity for workers; it also means exploitation. Workers are exploited because the surplus value — a worker creates more value in a day than he gets paid — is taken by the capitalist. Or, in other words: the wages of the laborer have a smaller exchange value than the exchange value of the object he or she produces. The object is sold by the capitalist, who buys labor and pockets the difference. A typical worker produces values that exceeds the reimbursement of their labor. A worker works more than the hours necessary to embody in his product the value of his labor power.
For example, if the value of labor power, i.e., the wage, is $50 a day, and a worker produces a good (or goods) which is worth $100 during a full day of work, then the second half of the day would yield surplus value, in this case another $50.
The expropriation of this surplus value is theft, because the capitalist takes something which he has not produced. He or she takes the unpaid labor and products of someone else and lives on the back of someone else, simply because of the privilege of owning the means of production. The workers have to accept this because they depend on the capitalist. They have to sell their labor power in order to survive because they do not own means of production and hence can’t produce and live without the consent of the capitalists. As the workers’ energy is not depleted after their own reproduction is guaranteed — through the payment of a wage — one can use it to produce more.
Moreover, the capitalist continuously tries to maximize this surplus value. There are two means to achieve this maximization. First, technology and science increase productivity and diminish the necessary labor time per unit of production. Machines make it possible to produce more with less labor. If wages stay constant and productivity goes up, then surplus value goes up.
But wages, of course, do not stay constant. The capitalist also tries to make labor as cheap as possible and the working day as long as possible. This is another kind of oppression, different from but inextricably linked to the expropriation of surplus value. This is achieved by way of the so-called “industrial reserve army”. This is a relatively large group, constantly available but not necessarily made up of the same people. They are perhaps unemployed, or working for peanuts in the developing world, or perhaps they are children. And they are desperate for (better paid) work, ready to replace the (elsewhere) employed and ready to accept a lower wage and a longer working day. This reserve army is a millstone round the neck of the proletariat, a regulator keeping wages at a low level.
The capitalist accumulates surplus value and wealth, and the worker accumulates misery. This “immiserization” (“Verelendung”) of the proletariat is something relative: it’s the gap in resources between those who own the means of production and those who don’t that will widen.
This maximization of surplus value deepens social divisions, brings despair to the workers, and hence will ultimately contribute to the collapse of capitalism, or so Marx believed.