Politicians often appeal to moral principles as a least-cost method of enforcing their policy demands. To do so effectively, they must understand how such principles fit into ordinary people's decision functions. Here [ distinguish three ways for formally representing moral principles. One reduces morality to enlightened self-interest, denying that morality has any special place in the decision calculus. Another, x~hile acknowledging that people do internalize moral principles per se, enters them into utility functions as just another consumption good. Truly strong moral principles, however, are best represented by a third model of seriously held moral principles which must be kept formally apart from mundane considerations. Such principles are as precarious as they are powerful. Policy-makers who want to tap them must respect the formalisms that make them strong, most typically by shielding moral principles from contamination by egoistic impulses. President Carter's declaration of "the moral equivalent of war" against the energy crisis is only the latest instance of a time-honoured strategy. Politicians everywhere have, from time to time, attempted to secure voluntary compliance with policies imposing heavy burdens upon people by appealing to their deepest moral sentiments.But before we can design social policies to play on people's better instincts, we must first understand how moral principles figure in ordinary people's decision calculi.Here I shall survey three models, one representing morality as enlightened selfinterest, another depicting it as internalized norms and a third focusing upon formally distinct "seriously-held" moral principles. Policy-makers would most like to evoke the very strong principles of this third category. But to do so they must respect the formalisms that give these principles their strength. In particular, they must avoid mixing moral and material incentives lest people be put off their principles altogether.