Karl Marx explained the whole phenomenon of exploitation of poor by the capitalist in the capitalist society by his theory of surplus value. He argued that surplus value is profit made by the capitalist by selling the objects at higher value than the investment for production of the object. In this way, workers who produce social objects are paid less than what they deserve. Capitalist stoles their labour for making their profit. 

Marx further argued that it is only in the class base societies where surplus value exists because bourgeois class, who own the land, capital and factories, exploits the proletariat, who has nothing. Marx thought that the gigantic increase in wealth and population from the 19th century onwards was mainly due to the competitive striving to obtain maximum surplus-value from the employment of labor. 

Consequently, it results in an equally gigantic increase of productivity and capital resources. To the extent that increasingly the economic surplus is convertible into money and expressed in money, the assessment of wealth is possible on a larger and larger scale.