Marx now states (“in anticipation”) the following: “the increase in the productive force does not in and by itself increase prices.” This is at first sight rather cryptic.

What Marx is not saying is that an increase in the productivity of labour will not increase the per-unit price of commodities: no one in the world of classical political economy would have thought that this could be the case. What he means, rather, is that an increase in the productivity of labour does not, “in and of itself”, increase the sum of the prices of the commodities. Given the length of the working day, how much labour-time is objectified in a mass of commodity product is insensible to the division of this labour-time into necessary labour and surplus labour. “The product [of one working day] objectifies one working day, whether the necessary time of labour is represented by 6 or 3 hours, by 1/2 or 1/4 of the working day.”

More (pdf, 120 KB): The Chapter on Capital: Part 11: Relative Surplus Value (II): The Increase in the Productivity of Labour and the Accumulation of Capital (pp. 341-347)