“Why these absurd questions?” asks the impatient reader. “Surely everyone knows that murder, assault, robbery, fraud, breach of contract, &c., are at variance with social welfare and must be punished when committed,” My replies are several. In the first place, I am quite content to have the questions called absurd; because this implies a consciousness that the answers are so self-evident that it is absurd to assume the possibility of any other answers. My second reply is that I am not desirous of pressing the question _whether_ we know these things, but of pressing the question _how_ we know these things. Can we know them, and do we know them, by contemplating the necessities of the case? or must we have recourse to “inductions based on careful observation and experience”? Before we make and enforce laws against murder, ought we to inquire into the social welfare and individual happiness in places where murder prevails, and observe whether or not the welfare and happiness are greater in places where murder is rare? Shall robbery be allowed to go on until, {220} by collecting and tabulating the effects in countries where thieves predominate and in countries where thieves are but few, we are shown by induction that prosperity is greater when each man is allowed to retain that which he has earned? And is it needful to prove by accumulated evidence that breaches of contract impede production and exchange, and those benefits to each and all which mutual dependence achieves? In the third place, these instances of actions which, pushed to extremes, cause social dissolution, and which, in smaller degrees, hinder social co-operation and its benefits, I give for the purpose of asking what is their common trait. In each of such actions we see aggression—a carrying on of life in a way which directly interferes with the carrying on of another’s life. The relation between effort and consequent benefit in one man, is either destroyed altogether or partially broken by the doings of another man. If it be admitted that life can be maintained only by certain activities (the internal ones being universal, and the external ones being universal for all but parasites and the immature), it must be admitted that when like-natured beings are associated, the required activities must be mutually limited; and that the highest life can result only when the associated beings are so constituted as severally to keep within the implied limits. The restrictions stated thus generally, may obviously be developed into special restrictions referring to this or that kind of conduct. These, then, I hold are _a priori_ truths which admit of being known by contemplation of the conditions—axiomatic truths which bear to ethics a relation analogous to that which the mathematical axioms bear to the exact sciences.