Political realism refuses to identify the moral aspirations of a particular nation with the moral laws that govern the universe. As it distinguishes between truth and opinion, so it distinguishes between truth and idolatry. All nations are tempted-and few have been able to resist the temptation for long-to clothe their own particular aspirations and actions in the moral purposes of the universe. To know that nations are subject to the moral law is one thing, while to pretend to know with certainty what is good and evil in the relations among nations is quite another. There is a world of difference between the belief that all nations stand under the judgment of God, inscrutable to the human mind, and the blasphemous conviction that God is always on one’s side and that what one wills oneself cannot fail to be willed by God also.
Intellectually, the political realist maintains the autonomy of the political sphere, as the economist, the lawyer, the moralist maintain theirs. He thinks in terms of interest defined as power, as the economist thinks in terms of interest defined as wealth; the lawyer, of the conformity of action with legal rules; the moralist, of the conformity of action with moral principles. The economist asks: “How does this policy affect the wealth of society, or a segment of it?” The lawyer asks: “Is this policy in accord with the rules of law?” The moralist asks: “Is this policy in accord with moral principles?” And the political realist asks: “How does this policy affect the power of the nation?” (Or of the federal government, of Congress, of the party, of agriculture, as the case may be.)
This realist defense of the autonomy of the political sphere against its subversion by other modes of thought does not imply disregard for the existence and importance of these other modes of thought. It rather implies that each should be assigned its proper sphere and function. Political realism is based upon a pluralistic conception of human nature. Real man is a composite of “economic man,” “political man,” “moral man,” “religious man,” etc. A man who was nothing but “political man” would be a beast, for he would be completely lacking in moral restraints. A man who was nothing but “moral man” would be a fool, for he would be completely lacking in prudence. A man who was nothing but “religious man” would be a saint, for he would be completely lacking in worldly desires.
[Hans J. Morgenthau, Politics Among Nations: The Struggle for Power and Peace]