Rigoristic Ethic displayed its strength against anti-ethical utilitarianism and anti-philosophical psychologism, not only in traditional scholastic, but also in the explicit polemic undertaken by Cudworth, Cumberland, Clarke and Price, against Hobbes, Locke, and the other utilitarians who followed them. The makers of great systems, too, attached themselves to ethical rigorism, Descartes (and in a certain sense Spinoza), Malebranche, Leibnitz, and the philosophy of the school of Leibnitz, as the moral consciousness declared itself in its true nature in Jean Jacques Rousseau against the French materialists. But rigorism also ended by contradicting itself in the same way as utilitarianism, owing to its one-sidedness, when it recognized a principle that was not merely utilitarian or that lost itself in mystery, either by reasoning with the utilitarian principle in the course of its development, or by receiving utilitarianism into itself, without any mediation, in the form of the morally indifferent. This is an old evil, which had already appeared in the _ἀδιάφορα_ of Stoicism, and in all those exceptions to the rigorous moral law, which ascetic Christianity had been obliged to allow, in order to exist side by side with the worldly life. [Sidenote: _Kant and his affirmation of the ethical principle._] III. The strength and the weakness of rigorism are to be clearly seen in the greatest ethical system to which it led: the moral doctrine of Emmanuel Kant. It was time that the principle of Christian Ethic should be reaffirmed, duty as clearly distinguished from pleasure, giving to it that relief which it had been without in the systems of Descartes and of Leibnitz, after the materialistic and utilitarian orgy that had lasted for more than a century, and after the equivocal attempts at an approach and fusion of the useful and the moral. Kant did not indeed in this respect oppose Wolffian Leibnitzianism; and although the ethical concept of _perfectio_ seemed to him to be empty and indeterminate, yet he was never able to prove that it was a eudæmonistic and utilitarian concept.[5] But that concept certainly had not the energy of duty and of the Kantian categoric imperative, which are true declarations of war against every heteronomous morality. This is the merit of Kant, after whom no serious philosopher can be anything but a Kantian in Ethic, as, after Christianity, to no one, not a wind-bag or an extravagant, is it given to be anything but a Christian. Moral action has no other motive than morality itself: to promote one's own happiness (said Kant) can never be _immediately_ duty, and even less the principle of all duties. [Sidenote: _Self-contradictions of Kant concerning the concept of the useful, of prudence, of happiness, etc._]