The Postan thesis is that medieval agriculture had low yields because there was insufficient pasture to keep the arable land fertile. This argument (and variants of it) has become an orthodox technological explanation for low preindustrial yields. Yet the thesis, on its face, implies that early cultivators were ignorant, irrational, or completely custom bound. This article develops a revised Postan thesis, in which medieval cultivators knew that pasture restored fertility but were unwilling to employ it. Impatience made this way of increasing yields unattractive because it required large capital investments in the soil nitrogen stock.