From Stanford’s Encyclopedia of Philosophy entry on Karl Marx:

Suppose that such commodities take four hours to produce. Thus the first four hours of the working day is spent on producing value equivalent to the value of the wages the worker will be paid. This is known as necessary labour. Any work the worker does above this is known as surplus labour, producing surplus value for the capitalist. Surplus value, according to Marx, is the source of all profit. In Marx’s analysis labour power is the only commodity which can produce more value than it is worth, and for this reason it is known as variable capital. Other commodities simply pass their value on to the finished commodities, but do not create any extra value. They are known as constant capital. Profit, then, is the result of the labour performed by the worker beyond that necessary to create the value of his or her wages. This is the surplus value theory of profit.

Read how Marx got this wrong here.

There is more:

Although Marx’s economic analysis is based on the discredited labour theory of value, there are elements of his theory that remain of worth. The Cambridge economist Joan Robinson, in An Essay on Marxian Economics, picked out two aspects of particular note. First, Marx’s refusal to accept that capitalism involves a harmony of interests between worker and capitalist, replacing this with a class based analysis of the worker’s struggle for better wages and conditions of work, versus the capitalist’s drive for ever greater profits. Second, Marx’s denial that there is any long-run tendency to equilibrium in the market, and his descriptions of mechanisms which underlie the trade-cycle of boom and bust. Both provide a salutary corrective to aspects of orthodox economic theory.

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