This article explores parallels between the debate prompted by Pareto's reformulation of choice theory at the beginning of the twentieth century and current controversies about the status of behavioural economics. Before Pareto's reformulation, neoclassical economics was based on theoretical and experimental psychology, as behavioural economics now is. Current 'discovered preference' defences of rational-choice theory echo arguments made by Pareto. Both treat economics as a separate science of rational choice, independent of psychology. Both confront two fundamental problems: to find a defensible definition of the domain of economics, and to justify the assumption that preferences are consistent and stable.