G.E. Moore According to G.E. Moore, Ethics is a "systematic science," that seeks to offer "correct reasons for thinking that this or that is good," (6). As a science and a rational discourse, Ethics must be based in logic and must not reflect any logical fallacies. Therefore, in sections 5-13 of Principia Ethica, Moore systematically analyzes the nature of 'good' and proves that it is logically impossible to define the term. In fact, Moore shows that one of the most common philosophical pitfalls is the attempt to define 'good,' either in absolute or relational terms. As Moore illustrates, good is indefinable because the concept is too simple and too fundamental. A definition necessarily implies a breaking down of something into relational parts; because the concept of 'good' cannot be broken down into any smaller elements, it cannot be defined logically. Because Ethics is a logical discourse, no philosopher studying Ethics can define 'good.'

Although good is indefinable, Moore ironically asserts that "how 'good' is to be defined is the most fundamental question in all Ethics," (5). Moore calls the question "What is good?" The "first question" of Ethics, and proposes that "Unless this first question be fully understood and its true answer clearly recognized, the rest of Ethics is as good as useless from the point-of-view of systematic knowledge," (5). Moore believes this for two reasons: one, because 'good,' however, indefinable, is central to the study of Ethics; and two, because Ethics is a "systematic science" dependent on logic. Interestingly, the answer to the question "What is good?" does not imply that good can be defined. Rather, the "true answer" that Moore wants his readers to recognize is that 'good' is an indefinable concept that is nevertheless the primary consideration of Ethics.

In fact, 'good' is to Ethics as 'foot' is to podiatry. However,...

Moore uses the analogy of a horse to illustrate why one term can be defined and not another. One can define a horse as a "hoofed quadruped of the genus Equus," for example, but one cannot define 'good' in this way. 'Good' is too simple, too pure of a concept to narrow down any further than it already is. Similarly, Moore points out that it would be impossible to define the color yellow because there is nothing to compare it to, nothing more fundamental than the appearance of the color itself. To define yellow in terms of its physics is useless because we cannot actually perceive the physical vibrations of color. Likewise, it is impossible to define 'good.'
One of the reasons why the "first question" is fundamental to Ethics is because the 'good' is the "only simple object of thought which is peculiar to Ethics," (5). No other discipline concerns itself so singularly with what is 'good' and what is 'not good.' 'Good' is the domain of Ethics much as 'God' is the domain of theologians and 'brain' is the domain of the neuroscientist. However, philosophers providing definitions of 'good' fail to ascribe to reason and logic, which Moore believes are necessary for Ethics to be a viable, practical "systematic science.'

Another reason why the "first question" is so important is that a mistaken definition of 'good' leads to more "erroneous ethical judgments than any other," (5). Ethical judgments must be based on reason; otherwise they are vacuous proscriptions. Moore states that the object of Ethics is "to give correct reasons for thinking that this or that is good," (6). Simply stating that "being nice is good" is not enough. The only way to reasonably deem one thing or another as 'good' is…