Although the obvious contradiction of the two forms of reason in Kant's teaching, the fundamental antagonism of pure and practical reason, was recognized and attacked at the very beginning of the century, it is still pretty widely accepted. The modern school of neo-Kantians urges a "return to Kant" so pressingly precisely on account of this agreeable dualism; the Church militant zealously supports it because it fits in admirably with its own mystic faith. But it met with an effective reverse at the hands of modern science in the second half of the nineteenth century, which entirely demolished the theses of the system of practical reason. Monistic cosmology proved, on the basis of the law of substance, that there is no personal God; comparative and genetic psychology showed that there cannot be an immortal soul; and monistic physiology proved the futility of the assumption of "free will." Finally, the science of evolution made it clear that the same eternal iron laws that rule in the inorganic world are valid too in the organic and moral world. But modern science gives not only a negative support to practical philosophy and ethics in demolishing the Kantian dualism, but it renders the positive service of substituting for it the new structure of ethical monism. It shows that the feeling of duty does not rest on an illusory "categorical imperative," but on the solid ground of _social instinct_, as we find in the case of all social animals. It regards as the highest aim of all morality the re-establishment of a sound harmony between egoism and altruism, between self-love and the love of one's neighbor. It is to the great English philosopher, Herbert Spencer, that we owe the founding of this monistic ethics on a basis of evolution. Man belongs to the social vertebrates, and has, therefore, like all social animals, two sets of duties--first to himself, and secondly to the society to which he belongs. The former are the behests of self-love or egoism, the latter of love for one's fellows or altruism. The two sets of precepts are equally just, equally natural, and equally indispensable. If a man desire to have the advantage of living in an organized community, he has to consult not only his own fortune, but also that of the society, and of the "neighbors" who form the society. He must realize that its prosperity is his own prosperity, and that it cannot suffer without his own injury. This fundamental law of society is so simple and so inevitable that one cannot understand how it can be contradicted in theory or in practice; yet that is done to-day, and has been done for thousands of years.