À Karl Marx, le 29 janvier 1851

Anyhow, your new thing about land rent is absolutely right. The increasing infertility of the land concomitant with an ever-increasing population in Ricardo has always seemed to me implausible, nor have I ever been able to discover any evidence in support of his ever-rising price of corn, but with my notorious sloth en fait de théorie, I have silenced the inward grumbling of my better self and have never gone to the root of the matter. There can be no doubt that you have hit on the right solution, […]

I have never really been able to accept Ricardo’s simple proposition in which he represents land rent as the difference in the productivity of various types of land and, seeking to prove this proposition, 1) acknowledges no other factor than the bringing under cultivation of ever poorer types of soil, 2) completely ignores advances in agriculture and 3) finally abandons almost entirely the bringing under cultivation of poorer types of soil, and instead continually proceeds from the assumption that capital, employed successively on a particular field, contributes less and less to the increase in the yield. Convincing as was the proposition to be proved, the factors adduced in proof of that same proposition were wholly alien to it. You will remember that, in the Deutsch-Französische Jahrbücher, I already invoked the progress made by scientific agriculture as against the theory of increasing infertility — of course very crude and not at all closely argued.