The practice of developed capitalism about the middle of the 19th century directly challenged proletarian thinkers to criticise Kant's doctrine of practical reason. Bourgeois ethics and freedom manifested themselves in the form of freedom of exploitation in the interest of the bourgeoisie, as slavery for the working class. The maintenance of human dignity appeared in reality as the brutalization and degradation of the proletarians, and the state founded on justice proved to be nothing but the class state of the bourgeoisie. And so it was seen that Kant's sublime ethics, instead of being the basis in all eternity of human activity in general, was merely the expression of the narrow class interests of the bourgeoisie. This proletarian criticism was the first material for a general theory, and once it had been stated, its correctness was demonstrated more and more by the study of previous historical events, and these events were thereby shown in their proper light. It was then understood by this theory that the social classes, distinguished by their position in the process of production, had different and antagonistic economic interests, and that each class did necessarily regard its own interest as good and sacred. These general class interests were not recognized in their true character but appeared to men in the guise of superior moral motives; in this form they crowded the special individual interests into the background, and since the class interests were generally felt, all the members of the same class recognized them. Moreover, a ruling class could temporarily compel a defeated or suppressed class to recognize the class interests of the rulers as a moral law, so long as the inevitability of the mode of production in which that class ruled was acknowledged. Owing to the fact that the nature and significance of the productive process was not understood, the origin of human motives could not be discovered. They were not traced back to experience, but simply felt directly and intuitively. And consequently they were thought to be of a supernatural origin and eternal duration. Not only the moral codes, but also other products of the human mind, such as religion, science, arts, philosophy, were then understood to be intimately connected with the actual material conditions of society. The human mind is influenced in all its products by the entire world outside of it. And thus the mind is seen to be a part of nature, and the science of the mind becomes a natural science. The impressions of the outer world determine the experience of man, his wants determine his will, and his general wants his moral will. The world around him determines man's wants and impressions, but these, on the other hand, determine his will and activity by which he changes the world; this will-directed activity appears in the process of social production. In this manner man by his work is a part, a link in the great chain of natural and social development.