The word “legitimacy” and its derivations (legitimate, legitimation, etc.) are widely employed in scientific language as they also are in current usage. In fact, we find them in several areas, from that of reasoning (“this conclusion is legitimate”) to that of law (“judgment of legitimacy”, “legitimate family”) and politics (“legitimate sovereign”). It is particularly in this latter domain, however, that they have their normal use as qualifications for power, and it is this particular aspect that I shall consider in this paper. In accredited political language “legitimacy” designates a principle, or criterion, of justification for power (acting and commanding) and, correlatively, of the obedience that is its due.