The present article claims an approximation to the criteria or legitimacy principles of the political power. For it, first a general approach of the problem is done; then, the text centres on the conception on the power and the legitimacy that Guglielmo Ferrero develops; finally, Ferrero's approach is criticized showing that for lacking a wide concept of reason he cannot give complete reasons of the democratic legitimacy. This work is based on two methodologies: Analytic and Normative. It concludes that only starting with a practical rationality from a moral stand point we may actually account for a criteria of legitimacy.