These, then, are the principal lines of development of the psychical integration. Common interest in maintaining order and law and peace produce a strong feeling of solidarity, which may be called “a consciousness of belonging to the same state.” (c) THE DIFFERENTIATION: GROUP THEORIES AND GROUP PSYCHOLOGY On the other hand, as in all organic growth, there develops _pari passu_ a psychic differentiation just as powerful. The interests of the group produce strong group feelings; the upper and lower strata develop a “class consciousness” corresponding to their peculiar interests. The separate interest of the master group is served by maintaining intact the imposed law of political means; such interest makes for “conservatism.” The interest of the subject group, on the contrary, points to the removal of the prevailing rule, to the substitution for it of a new rule, the law of equality for all inhabitants of the state, and makes for “liberalism” and revolution. Herein lies the tap root of all class and party psychology. Hence there develop, in accordance with definite psychological laws, those incomparably mighty forms of thought which, as “class theories,” through thousands of years of struggle guide and justify every social contest in the consciousness of contemporaries. “When the will speaks reason has to be silent,” says Schopenhauer, or as Ludwig Gumplowicz states the same idea, “Man acts in accordance with laws of nature, as an afterthought he thinks humanly.” Man’s will being strictly “determined,” he must act according to the pressure which the surrounding world exerts upon him; and the same law is valid for every community of men: groups, classes, and the state itself. They “flow from the plane of higher economic and social pressure to that of lower pressure, along the line of least resistance.” But every individual and each community of men believe themselves free agents; and therefore, by an unescapable psychical law they are forced to consider the path they are traversing as a freely chosen means, and the point toward which they are driven as a freely chosen end. And since man is a rational and ethical being, that is, a social entity, he is obliged to justify before reason and morality the method and the objective point of his movement, and to take account of the social consciousness of his time. So long as the relations of both groups were simply those of internationally opposed border enemies, the exercise of the political means called for no justification, because a man of alien blood had no rights. As soon, however, as the psychic integration develops, in any degree, the community feeling of state consciousness, as soon as the bond servant acquires “rights,” and the consciousness of essential equality percolates through the mass, the political means requires a system of justification; and there arises in the ruling class the group theory of “legitimacy.”