Ethics is a science of relations. The things related are human interests, human ends. The ideal which ethics proposes to itself is the unity of ends, just as the ideal of science is the unity of causes. The ends of the natural man are the subject-matter with which ethics deals. The ends of the natural man are not to be crushed or wiped out, but to be brought into right relations with one another. The ends of the natural man are to be respected from an ethical point of view, so long as they remain within their proper limits. The moral laws are formulas expressing relations of equality or subordination, or superordination. The moral virtue of our acts consists in the respect which we pay to the system of relationships thus prescribed, in the willingness with which we co-ordinate our interests with those of others, or subordinate them to those of others, as the exigencies of the moral situation may require. But the point on which it is now necessary to fix our attention is that when morality has once sanctioned any of the ends of life, the natural man may be left to pursue them without interference on the part of the moralist. When morality has marked out the boundaries within which the given end shall be pursued, its work so far is done; except, indeed, that we are always to keep an eye upon those boundaries, and that the sense of their existence should pervade the whole atmosphere of our lives.[3] A few illustrations will make my meaning clear. There is a moral rule which says that we should eat to live; not, conversely, live to eat. This means that we should regulate our food in such a way that the body may become a fit instrument for the higher purposes of existence, and that the time and attention bestowed upon the matter of eating shall not be so great as to divert us from other and more necessary objects. But, these limits being established, it does not follow that it is wrong or unspiritual to enjoy a meal. The senses, even the lowest of them, are permitted to have free play within the bounds prescribed. Nor, again, should we try rigidly to determine the choice of food according to moral considerations. It would be ridiculous to attempt to do so. The choice of food within a wide range depends entirely on taste, and has nothing to do with moral considerations (whether, for instance, we should have squash or beans for dinner). Those who are deeply impressed with the importance of moral rules are often betrayed into applying them to the veriest minutiae of conduct. Did they remember that ethics is a science of relations, or, what amounts to the same thing, a science of limits, they would be saved such pedantry. Undoubtedly there are moral _adiaphora_. The fact that such exist has been a stumbling-block in the way of those who believe that morality ought to cover the whole of conduct. The definition of ethics as a science of relations or limits removes this stumbling-block. Ethics stands at the frontier.