"In the social production of their lives, men enter upon certain definite, necessary relations, relations independent of their will, relations of production that correspond with definite degrees of development of their material productive forces. The totality of these relations of production constitute the economic structure of society, the true basis from which arises a juridical and political superstructure to which definite social forms of consciousness correspond. The mode of production of material life determines the social, political and intellectual processes of life. It is not the consciousness of mankind that determines their being, but, on the contrary, their social being that determines their consciousness. In a certain stage of their development, the material forces of production of society come into contradiction with the existing relations of production, or, which is only a juridical expression for the same thing, with the relations of property within which they had hitherto moved. From forms for the development of these forces of production, they are transformed into their fetters. We then enter upon an epoch of social revolution."[3] This completely materialist conception of history is one of the greatest discoveries of our century, so rich in scientific discoveries. Thanks to it alone sociology has at last, and for ever, escaped from the vicious circle in which it had, until then, turned; thanks to it alone this science now possesses a foundation as solid as natural science. The revolution made by Marx in social science may be compared with that made by Kopernicus in astronomy. In fact, before Kopernicus, it was believed that the earth remained stationary, while the sun turned round it. The Polish genius demonstrated that what occurred was the exact contrary. And so, up to the time of Marx, the point of view taken by social science, was that of "human nature;" and it was from this point of view that men attempted to explain the historical movement of humanity. To this the point of view of the German genius is diametrically opposed. While man, in order to maintain his existence, acts upon nature outside himself, he alters his own nature. The action of man upon the nature outside himself, pre-supposes certain instruments, certain means of production; according to the character of their means of production men enter into certain relations within the process of production (since this process is a social one), and according to their relations in this social process of production, their habits, their sentiments, their desires, their methods of thought and of action, in a word, their nature, vary. Thus it is not human nature which explains the historical movement; it is the historical movement which fashions diversely human nature.