Politics require principles. When it is a question on the one hand of acquiring a position, and on the other hand of defending it, the opposed interests seek bases upon which they can rest. The sovereignties feel the necessity of invoking the authority of a principle which seems to draw its force from considerations superior to those that can inspire the political pretensions of a particular state. An appeal is made to a common interest the supposed existence of which is taken for granted—namely, that of maintaining the peace and good relations among the states—and it is presented in a definite formula, the principle of equilibrium.