The same Latin word,"ratio", is at the origin of two distinct traditions in Western thought. For the moralists reason was opposed to passion and, among more recent writers, to interest. For modern economists, rationality, although compatible with the pursuit of interest, is opposed to passion and to irrational behavior more generally. In a positive characterization, action based on reason requires impartial motives, with respect both to individuals and to moments of time, and well-grounded beliefs. Rational action, while neutral with respect to motives, also requires well-grounded beliefs, although the criteria for well-groundedness may differ. Reason provides objective grounds for action (“external reasons�), while rationality offers subjective grounds (“internal reasons�). In a final contrast, the task of the preceptor of the prince is to inculcate reason, that of the advisor to the prince to spell out the requirements of instrumental rationality for achieving his aims, whatever they might be. The "mild voice of reason� (Madison) is strenghtened by the fact that in most societies it occupies a privileged place in the normative hierarchy of motivations. Even those ultimately moved by interest or passion, may want to present their actions, to themselves or to others, as moved by reason. Much of the impartial behavior we observe may be due to this civilizing force of hypocrisy and of self-deception. Rationality and amour-propre may be the handmaidens of reason.