But our question What is good? may still have another meaning. We may, in the third place, mean to ask, not what thing or things are good, but how good is to be defined. This is an enquiry which belongs only to Ethics, not to Casuistry; and this is the enquiry which will occupy us first. (§ 5 ¶ 1)

It is an enquiry to which most special attention should be directed; since this question, how good is to be defined, is the most fundamental question in all Ethics. That which is meant by good is, in fact, except its converse bad, the only simple object of thought which is peculiar to Ethics. Its definition is, therefore, the most essential point in the definition of Ethics; and moreover a mistake with regard to it entails a far larger number of erroneous ethical judgments than any other. Unless this first question be fully understood, and its true answer clearly recognised, the rest of Ethics is as good as useless from the point of view of systematic knowledge. True ethical judgments, of the two kinds last dealt with, may indeed be made by those who do not know the answer to this question as well as by those who do; and it goes without saying that the two classes of people may live equally good lives. But it is extremely unlikely that the most general ethical judgments will be equally valid, in the absence of a true answer to this question; I shall presently try to shew that the gravest errors have been largely due to beliefs in a false answer. And, in any case, it is impossible that, till the answer to this question be known, any one should know what is the evidence for any ethical judgment whatsoever. But the main object of Ethics, as a systematic science, is to give correct reasons for thinking that this or that is good; and, unless this question be answered, such reasons cannot be given. Even, therefore, apart from the fact that a false answer leads to false conclusions, the present enquiry is a most necessary and important part of the science of Ethics. (§ 5 ¶ 2)