This is not to be interpreted as meaning that aggression is permissible as long as retaliation is permitted; for the formula means that interference with another's life is limited, that life shall not be impeded in one case further than is necessary to the maintenance of other lives; it does not countenance a superfluous interference on the ground that an equal interference may balance it. In earlier stages, the conception of justice was this erroneous one of a balancing of injuries--an eye for an eye and a tooth for a tooth. By oscillations which become gradually less, social equilibrium is approached; and with this approach to equilibrium comes approach to a definite theory of equilibrium. In the reigning school of politics and morals, scorn is expressed for every doctrine which implies restraint of immediate expediency, or what appears to be such;--contempt for generalizations and abstract principles, with unlimited faith in political machinery. Strangely enough, we find this approval of political empiricism and disbelief in any other guidance, in the world of science also. The accepted scientific fact that causation holds of the actions of incorporated men as of other parts of nature, remains a dead letter; there is no attempt to identify the causation, and ridicule is visited upon those who endeavor to find a definite expression for the fundamental principle of harmonious social order. Peoples with whom confusion is not caused by the conflicting disciplines of outer war and internal peace, early arrive at the principle of equity, and accordingly some uncivilized tribes show a stronger sense of it than is found among civilized peoples. Nevertheless, the conception of justice has slowly evolved to some extent, and is expressed in such formulae as, "Do unto others as ye would that others should do unto you" (too sweeping a statement of the equality of claims, since it implies no recognition of the inequality necessary in the shares of good respectively appropriate), or in the Kantian rule, which is an allotropic form of the Christian rule. Jurists, too, have recognized a natural law of equity underlying human law. To the reproach that belief in such a law is an _a priori_ belief, it may be answered that _a priori_ beliefs are explained by the theory of evolution, as arising with determination of the nervous system and certain resulting necessities of thought, and that they differ from _a posteriori_ beliefs merely in the circumstance "that they are the products of the experiences of innumerable successive individuals, instead of the experiences of a single individual." If we ask for the ground of the greatest happiness principle, we come to an _a priori_ belief also; for whence is the postulate? If it is an induction, where and by whom has the induction been drawn; and if it is a truth of experience derived from careful observation, then what are the observations, and when was