Biologists recognize that organisms adjust choice when constraints change, so-called “phenotypic plasticity.” Economists call it “rationality.” But what is the origin of rationality? Neo-Darwinists conceive rationality as a trait. But this cannot be the case. Let us suppose two lineages of rationality, R1 and R2. Natural selection would supposedly favor R1 over R2 under C1 constraints and vice versa under C2 constraints. However, if agents are using different rationalities, the fitness functions are incommensurable. For them to be commensurable, there must be only a single kind of rationality, R. But how could R=R1 and R=R2, when R1 and R2 are different?