Man, even the most heroic, is not the sovereign maker and law-giver of social life, but its executive; he only follows out the tendencies and currents set up by the material foundation of society. Nevertheless, a great deal depends upon the executive officials. If they possess comprehensive knowledge, energetic natures, and outstanding capacities, they are able, within the boundaries drawn for them, to accomplish great things, and to accelerate the development. "Up to the present the philosophers have but interpreted the world; it is, however, necessary to change it."--(Marx, "Theses to Feuerbach.") We have referred in various places to interests. We are not to understand by this personal, but general social or class interests. Marx is not of the opinion that everybody acts in accordance with his personal welfare. This is not Marxian doctrine, but that of the middle-class moral philosophers, like Helvetius (1715-1771) and Jeremy Bentham (1748-1832), who regarded pleasure and pain of the individual as the measure and motive of his actions and conduct. Marx is rather of the opinion that men often, in the most important events of their lives, act contrary to their personal interests, as in their feelings and thoughts they identify themselves with that which they hold to be the interests of the community or of their class. According to Marx, individual interest generally plays a slight part in history. He is preoccupied with the collective interest of social production. Only the latter does he hold to be determining in the formation of the intellectual superstructure. Up till now we have only spoken of various forms of production and society, and their corresponding mental systems. But we do not yet know why and how one form of production and society becomes obsolete and gives place to another, that is, how and why revolutionary changes are brought about. Or in other words: we have hitherto considered the statics of society; we will now look at its dynamics. The revolutionary changes in society depend on two groups of phenomena, which, although casually connected with each other, yet work differently. One of these groups of phenomena is technical, and consists in changes in the productive forces. The other group, which is the effect of the first, is of a personal nature, and consists in struggles between the social classes. Let us consider the first group of causes.