Marx's project: where do profits come from?

A main task of a theory of value for capitalist economy, for Marx, was to answer the question: Where do profits come from? Perhaps it is easier to comprehend why this question was of a somewhat paradoxical nature by comparing capitalism to the two major precapitalist, classstratified modes of production: feudalism and slavery. Under slavery, the class of slave owners forcibly expropriated unpaid labor from slaves. Thus, slave owners lived off slaves in a most obvious way. Under the feudal organization of production, the expropriation of the surplus was almost as obvious. Serfs performed the corvée for several days a week and worked several days a week on their own plots. The product of their plots was theirs for consumption; the labor time spent on the corvée was transformed into goods expropriated directly by the lord. Again, there can be no confusion concerning the locus of surplus production, and the locus of its expropriation. In both slavery and feudalism, the key to the division of society into a rich, expropriating class and a poor, expropriated one was the existence of a coercive institution for the exchange of labor.

What was puzzling to Marx concerning capitalism was this: The institution of labor exchange was not coercive. Nevertheless, one class became incredibly rich, and the other remained impoverished. Marx insisted on modeling capitalism as a regime in which markets are fair, an idea he tried to capture in his value theory by requiring commodities of equal value to be exchanged for each other.