The marginalist theory

Marx was an exponent of the 'Labour theory' of value, held also by Adam Smith, Ricardo, James Mill, John Stuart Mill and some early 19th century political economists. This is the theory that human labour is the sole source of value (of exchange value), and that the value of a commodity is the quantity of socially necessary labour embodied in it. When we exchange commodities they exchange in the ratio of the quantities of labour required to produce them and bring them into the market