Mill begins his essay by observing that very little progress has been made toward developing a set of standards by which to judge moral right and wrong. For more than two thousand years, people have been attempting to determine the basis of morality, but have not come any closer to consensus. Mill acknowledges that in the sciences, it is common to have disagreement about such bases or foundations. However, he argues that in science particular truths can still have meaning even if we do not understand the principles underlying them; in contrast, in areas such as law or ethics, a statement unfounded upon a generally accepted theoretical basis has very little validity at all. In these areas (unlike in science), all action exists to forward a particular end; thus it would seem that rules of action would depend on what ends are being pursued. Mill therefore argues that in order to know what morality dictates, it is necessary to know by what standard human actions should be judged.
Mill then addresses the issue of moral instinct, and whether the existence of such an instinct would eliminate the need for determining the foundation of morality. He argues it does not. First, the existence of such a moral sense is disputable. Secondly, even if this sense does exist, it does not tell us whether something is right or wrong in a particular case. Rather, this instinct supplies only general principles. Thus, although general laws are a necessary part of moral thinking, it is the application of these laws to specific cases that constitutes morality itself. However, people do not often try to make a list of these general laws, or a priori principles, that are the foundation of morality; nor do they attempt to reduce these to a single first principle. Rather, they either assume that commonly accepted moral rules should be seen as having a priori legitimacy, or they arbitrarily posit some implausible first principle that does not then gain popular acceptance. Mill argues that the moral claims made by many previous thinkers are therefore unfounded.
Yet our moral beliefs have undergone little alteration over the course of history; their durability implies that there exists some standard that serves as a solid, if unrecognized, foundation. Mill argues that this unrecognized standard is the principle of utility, or the "greatest happiness principle." He notes that utilitarianism has had tremendous influence in shaping moral doctrines, even among those people who reject the principle, such as Immanuel Kant.
Mill writes that his essay will reflect his attempt to add to the understanding and appreciation of utilitarianism, and to present some kind of proof of it as a moral theory. Utilitarianism cannot be "proven" in the ordinary sense of the word, Mill asserts, since it is not possible to prove questions regarding ultimate ends. Rather, the only statements that can be proven to be valid are those statements that lead to other statements that we accept to be valid. However, this does not mean that we must judge first principles arbitrarily; we can still evaluate them rationally. This essay, then, will present and consider various arguments in support of utilitarianism. Also, since much of the opposition to utilitarianism issues from misunderstandings of the theory, Mill says he will also focus on what utilitarianism actually posits.
In these introductory remarks, Mill sets the stage for his essay. It is helpful to observe his strategy of argument here. He begins by observing something of a crisis in moral thinking: basically, people have been unable to come to any consensus on what principles the notions of "right" and "wrong" are based on. Mill argues that having such a foundation is necessary in order for morality to have any legitimacy or significance. If actions are to be judged by whether they further "good" ends, it is necessary to know which ends are good. Moreover, the stakes of this question are high: it is not simply an academic debate; rather, legal and ethical thinking depend upon a clearly defined moral standard. Having presented this problem, Mill introduces utilitarianism as a potential solution. He argues that it is already implicitly used as a standard, and that it fulfils the requirements of being a first principle.
It is important to note that Mill defines morality's purpose as bringing about a particular state of the world. This is one framework through which to understand morality, and Mill defines it as the essential one. It is important to think about whether this consequences-based understanding of morality is convincing. For example, consider something regarded as immoral, such as lying. Consider then a situation in which the telling of a lie could prevent five other people from having to lie. Is the first lie morally justified? The answer depends in part on whether one believes that morality's essential function is to bring about the "best," general state of the world, or whether its function is to govern individual acts independent of their more general consequences: if one believes that the point of morality is to create a better world as a whole, and if you accept that lying is bad, then the fewer total lies in the world the better, and one should tell that first lie to prevent the other five from being told. Other accounts of morality, however, might argue that bringing about the best state of the world at large is not morality's concern. For example, one could argue that morality bears most strongly upon the conduct of a single person as an individual: as an individual, one should never lie, no matter what; to lie is to defile oneself morally. There are many variations of this argument, as well as completely different ways to potentially ground morality: Mill's view of morality is only way of considering the question.