G.E. Moore’s Principia Ethica is concerned with applying logic to ethics, and with demonstrating that logic can give ethics a better foundation. Moore defines ethics as an inquiry into what is good, including what is good in human conduct. Moore shows how false premises about the way in which good is to be defined can lead to false conclusions about ethical conduct.

Principia Ethica has six chapters. Chapter I is entitled “The Subject-Matter of Ethics,” Chapter II “Naturalistic Ethics,” Chapter III “Hedonism,” Chapter IV “Metaphysical Ethics,” Chapter V “Ethics in Relation to Conduct,” and Chapter VI “The Ideal.”

The outline of each chapter is given in the table of contents. Each chapter is divided into numbered sections. Chapter I consists of sections 1-23, Chapter II sections 24-35, Chapter III sections 36-65, Chapter IV sections 66-85, Chapter V sections 86-109, and Chapter VI sections 110-135.

Moore says that the subject-matter of ethics is most often concerned with human conduct, and with the question of what is good or bad, what is right or wrong. Thus, the fundamental question of ethics is how ‘good’ is to be defined. Moore argues that ‘good’ cannot be defined. He says that it cannot be analyzed, because it is a simple object of thought and not a complex object which can be divided into parts.

To determine what is good is to determine what has intrinsic value. But the investigation of intrinsic value is complicated by the fact that a complex object may have parts which are good, bad, or indifferent. The value of a whole object may not be the same as the values of its parts.

The ‘naturalistic fallacy’ is to assume that if we name various properties of things which we believe to be good, we are actually defining ‘good.’ Moore argues that 'Naturalism' does not provide any logical reason for any ethical principle, because it falsely assumes that it has defined what is good.

If we falsely assume that good can be defined, then good can become a property of things, and we have only to discover the characteristics of this property. If good is simply given another name, such as pleasure, or the object of desire, we cannot prove that any such name is better than any other. But if we ultimately recognize that good cannot be defined, then we realize that we must be more careful to find logical reasons for ethical principles.

There is an important difference between saying that something is a means to good, and saying that something is good in itself. This becomes more important when we say that something in itself has the property which we are asserting to belong to its effects. In determining whether an action is good, we need to ask not only how far this action is good in itself, but how far it tends to produce a good effect.

To judge whether a particular action is a means to good, we need to know not only that the action will produce a certain effect, but that the effect itself will be good. We need to know whether the action is the best means to achieve a good effect, and whether the action will produce a better effect than if some other action were performed.

The ‘naturalistic fallacy’ occurs when we think of something as good because the thing in question is related to some other natural object which we think of as good.

The natural object which we think of as good may be an object of experience, or it may be an object which is inferred to exist as a metaphysical reality. Pleasure is an example of a natural object which may be thought of as good. A Supreme Good may be thought to exist as a metaphysical reality.

Good may or may not be something ‘natural.’ Something that is ‘natural’ may or may not be good. To argue that something is good because it is ‘natural’ or bad because it is ‘unnatural’ is an example of the naturalistic fallacy (Chapter II, Section 29).

Moore says that 'Naturalistic Ethics' are characterized by the naturalistic fallacy. Naturalistic Ethics are seen in Hedonism and in 'Evolutionistic Ethics.'

'Evolutionistic Ethics' are characterized by the naturalistic fallacy in that they assume that the evolution of nature can be used to determine what is good. Moore says that there is no evidence that nature necessarily evolves toward good. To be ‘better’ does not necessarily mean to be more evolved; to be more evolved does not necessarily mean to be ‘better’ (Chapter II, Section 35).

Hedonism is characterized by the naturalistic fallacy. A fundamental principle of Hedonism is that pleasure is the highest good. An action that produces pleasure is a good action. An action that produces pain instead of pleasure is a bad action.

Hedonists act on the assumption that whatever leads to pleasure is good. Other things which are desired, such as virtue or knowledge, are good only as a means to pleasure, or for the sake of pleasure, and not as ends in themselves (Chapter III, Section 38).

Hedonism asserts that pleasure is good, and that pleasure is what is desired. We desire something because it causes pleasure. Happiness is pleasure, and the absence of pain.

Moore argues that if pleasure is considered good as an end in itself, then it must be good whether we are conscious of it or not. We do not need to know whether we are happy, if the consciousness of pleasure is not an end in itself. But how can we have pleasure if we do not know when we are happy? This problem exposes the misleading assumptions of Hedonism, Moore says.

Hedonism is in error if it manifests the fallacy of confusing the means and end. It is false to assume that pleasure and the consciousness of pleasure are the same, and that both pleasure and the consciousness of pleasure are good as ends in themselves.

Moore argues that, even if pleasure is considered as a means to an end, and not as an end in itself, the consciousness of pleasure cannot be considered as an end in itself, because in order to be truly pleasurable, the consciousness of pleasure must be combined with consciousness of other things (Chapter III, Section 57).

Moore asserts that Hedonism is present in Egoism and Utilitarianism. Egoism is a philosophy that each person should act to promote his or her own happiness, and that for each person, his or her own happiness is the highest good.

Moore says that Egoism, as it concerns the consequences of actions, is a philosophy that each person desires his or her own happiness, and that therefore each person’s happiness is the only thing desirable. But this is a form of contradiction, Moore says. To say that the happiness of one person is the only thing desirable, and that everyone’s happiness is the only thing desirable, is contradictory.

Egoism as a doctrine of means, on the other hand, says that each person’s happiness is a means to something else, and not an end in itself. Egoism as a doctrine of means may be seen in Utilitarianism. To be good, an action must produce the greatest balance of happiness over unhappiness, thus producing happiness for the greatest number of persons.

Utilitaranism affirms that actions are right insofar as they promote happiness, and wrong insofar as they promote unhappiness. The value of an action is judged by its consequences. Moore argues that Utilitarianism is contradictory, in that it does not accurately distinguish between actions that promote happiness only as a means to future happiness, and actions that promote happiness as an end in itself.

Utilitarianism may consider present happiness as a means to future happiness, but this may neglect the question of whether present happiness is an end in itself. Moore explains that if each person’s happiness is a means to happiness for the greatest number of persons, then each person’s own happiness cannot be an end in itself. This reveals another false assumption in Utilitarianism, Moore says.

Metaphysical Ethics reveal the naturalistic fallacy in that they assume that, by making propositions about the nature of ultimate reality, we can define what is good. Moore argues that Metaphysical Ethics do not accurately distinguish between the practical and the theoretical, between what is good and what should be good. The proposition that a metaphysical reality exists, and that it is good, is not distinguished from the proposition that a metaphysical reality logically should exist, and that it logically should be good.

Ethics in relation to human conduct is concerned with the question of what actions are right, and what actions are wrong. Moore says that actions may be causes or necessary conditions for what is good in itself. The question is then whether any given action is a means to good.

Moore argues that if something is good in itself, it has an intrinsic value, existing absolutely by itself. If something is intrinsically good, it is good independently of anything else. If its goodness derives from its being a part of something else, then it cannot be defined as good in itself.

Moore says that there are two things which are generally regarded as good in themselves: 1) personal affection, and 2) the appreciation of beauty in art or nature. These two forms of good may be combined to form an even greater good.

Personal affection, and the appreciation of beauty, may also become greater when they are related to a particular person or object existing in reality, or when they are related to the reality of a particular person or object. Thus, perfect knowledge, like perfect love, may be an aspect of the Ideal, or the highest good.

Moore says that, while knowledge has little or no value by itself, it is a necessary component of the highest good. Knowledge of the reality of the world contributes to the appreciation of what is good.

Knowledge can also be a means to good. Personal affection, and the appreciation of beauty, are increased by the knowledge of how they are related to reality.

Knowledge may cause us to feel an ethical duty to act toward the highest good. This is because the highest good is the rational end of human action.