Ethics is traditionally a department of philosophy, and that is my reason for discussing it. I hardly think myself that it ought to be included in the domain of philosophy, but to prove this would take as long as to discuss the subject itself, and would be less interesting.
As a provisional definition, we may take ethics to consist of general principles which help to determine rules of conduct. It is not the business of ethics to say how a person should act in such and such specific circumstances; that is the province of casuistry. The word 'casuistry' has acquired bad connotations, as a result of the Protestant and Jansenist attacks on the Jesuits. But in its old and proper sense it represents a perfectly legitimate study. Take, say, the question: In what circumstances is it right to tell a lie? Some people, unthinkingly, would say: Never! But this answer cannot be seriously defended. Everybody admits that you should lie if you meet a homicidal maniac pursuing a man with a view to murdering him, and he asks you whether the man has passed your way. It is admitted that lying is a legitimate branch of the art of warfare; also that priests may lie to guard the secrets of the confessional, and doctors to protect the professional confidences of their patients. All such questions belong to casuistry in the old sense, and it is evident that they are questions deserving to be asked and answered. But they do not belong to ethics in the sense in which this study has been included in philosophy.
It is not the business of ethics to arrive at actual rules of conduct, such as: 'Thou shalt not steal.' This is the province of morals. Ethics is expected to provide a basis from which such rules can be deduced. The rules of morals differ according to the age, the race, and the creed of the community concerned, to an extent that is hardly realized by those who have neither travelled nor studied anthropology. Even within a homogeneous community differences of opinion arise. Should a man kill his wife's lover? The Church says no, the law says no, and common sense says no; yet many people would say yes, and juries often refuse to condemn. These doubtful cases arise when a moral rule is in process of changing. But ethics is concerned with something more general than moral rules, and less subject to change. It is true that, in a given community, an ethic which does not lead to the moral rules accepted by that community is considered immoral. It does not, of course, follow that such an ethic is in fact false since the moral rules of that community may be undesirable. Some tribes of head-hunters hold that no man should marry until he can bring to the wedding the head of an enemy slain by himself. Those who question this moral rule are held to be encouraging licence and lowering the standard of manliness. Nevertheless, we should not demand of an ethic that it should justify the moral rules of head-hunters.
Perhaps the best way to approach the subject of ethics is to ask what is meant when a person says: 'You ought to do so-and-so' or 'I ought to do so-and-so.' Primarily, a sentence of this sort has an emotional content; it means 'this is the act towards which I feel the emotion of approval'. But we do not wish to leave the matter there; we want to find something more objective and systematic and constant. The ethical teacher says: 'You ought to approve acts of such-and-such kinds.' He generally gives reasons for this view, and we have to examine what sorts of reasons are possible. We are here on very ancient ground. Socrates was concerned mainly with ethics; Plato and Aristotle both discussed the subject at length; before their time, Confucius and Buddha had each founded a religion consisting almost entirely of ethical teaching, though in the case of Buddhism there was afterwards a growth of theological doctrine. The views of the ancients on ethics are better worth studying than their views on (say) physical science; the subject has not yet proved amenable to exact reasoning, and we cannot boast that the moderns have as yet rendered their predecessors obsolete.
(From An Outline of Philosophy, by Bertrand Russell.)