The second theory Marx propounded, of great influence, was the theory of exploitation under capitalism. Marx motivates the need for such a theory by observing that in slave and feudal modes of production, there is no mystery as to the source of the economic surplus and its acquisition by a small class of slave owners or feudal lords and roy-alty. Direct military and police suppression of slaves and serfs enabled the lords to appropriate the surplus, leaving only a subsistence consumption for the producers. But capitalism is different: nobody forces the worker to sell his labor power to the capitalist; labor power is traded on a competitive market (at least in the ideal form). Because the system is based upon voluntary trade, with de jure personal freedom, it is somewhat of a mystery how vast wealth accumulates in the hands of a small class, while the direct producers remain impoverished. After all, the direct coercion of workers characteristic of slave and feudal modes has been abolished. Marx proposed the theory of exploitation to solve the mystery.

That theory explained the mechanism with which capitalism accomplished the sleight-of-hand of concentrating the economic surplus in the hands of a small class of capitalists, under conditions of personal freedom and freedom to contract, and the theory of historical materialism conjectured that class society would be eliminated, once and for all, when capitalist property relations became a fetter on the further development of the" productive forces." As is well known, Marx wrote almost nothing about socialism, and it was left for later Marxists to formulate the details of how the economy would be organized after the revolution.