 Although social choice theory in this way bears on questions of social rationality, most work in social choice theory explores the consequences of principles of rationality coupled with explicitly ethical constraints. The seminal contribution is Kenneth Arrow’s impossibility theorem (1963, 1967). Arrow assumes that both individual preferences and social preferences are complete and transitive and that the method of forming social preferences (or making social choices) issues in some social preference ranking or social choice for any possible profile of individual preferences. In addition, Arrow imposes a weak unanimity condition: if everybody prefers X to Y, then Y must not be socially preferred. Third, he requires that there be no dictator whose preferences determine social preferences or choices irrespective of the preferences of anybody else. Lastly, he imposes the condition that the social preference between X and Y should depend on how individuals rank X and Y and on nothing else. Arrow then proved the surprising result that no method of relating social and individual preferences can satisfy all these conditions!