Mill begins this chapter by saying that it is not possible to prove any first principles by reasoning. How, then, can we know that utility is a foundational principle? The purpose of this chapter is to explore what should be required of utilitarianism in order for it to be believed as valid. Mill argues that the only proof that something is desirable is that people actually desire it. It is a fact that happiness is a good, because all people desire their own happiness. Thus, it is clear that happiness is at least one end, and one criterion, of morality.
However, in order to show that happiness is the sole criterion for morality, it is necessary to show that people never desire anything but happiness. Mill says that people do desire things like virtue, which in common language is distinguished from happiness. However, Mill states that people love virtue only because it constitutes a part of happiness. Mill argues that happiness is not an abstract idea, but a whole with component parts. Because virtue is a part of happiness, and promotes the general happiness, utilitarianism encourages the development of virtue.
Anything that is desired beyond being a means to happiness is desired because it is part of happiness. Thus, Mill explains that proving utilitarianism is a psychological question. The real issue is whether it is true that people only desire things that are part of happiness or a means to happiness. This can only be answered by self-reflection and observation of others. Mill contends that utilitarianism is true, and that impartial reflection will show that desiring something is the same thing as thinking it pleasant. He argues that this is so obvious that he doubts it could be disputed. The only possible refutation that could legitimately be made is that the moral will is something different than physical or emotional desire; virtuous people carry out actions without thought of such pleasures. Mill admits that will is different than desire, and often becomes an end in itself. However, all will originates in desire; if we will a thing that we now no longer desire, it is only by force of habit. This does not change the fact that things are good to people only insofar as they lead to pleasure. Mill then says that it leaves it to the "thoughtful reader" whether what he has said is true.
Mill further expands his discussion of happiness in this chapter. Recall that in Chapter 2, Mill argued that pleasures that were based on one's higher faculties were of a higher quality, and should be weighted accordingly. In this way, he tried to expand the meaning of happiness to allow for different kinds of pleasure. In Chapter 4 Mill expands the meaning of happiness again. A possible objection to utilitarianism is that certain experiences could be integral parts of a compound happiness, not merely a means to a pure, elemental happiness. Correspondingly, Mill argues now that utilitarianism can leave room for the fact that happiness consists of the other experiences that people value. This idea of happiness as having "component parts" is an important expansion of the meaning of happiness by Mill.
The other major argument in this chapter is that the motivation for all action is based on the fulfillment of desire. However, he probably rightly contends that whether he is correct is an empirical question, a question answered by observing oneself and others. This brings up an important question about the lines between psychology and philosophy. If utilitarianism is based on the psychological make-up of human beings, then to what degree is it merely descriptive? We tend to want philosophy to provide reasons why we should behave in a particular manner. However, to note that we do behave in a certain manner is not necessarily to prove that we ought to behave that way. One should consider at which points in the text Mill is observing how humans view the world, and at which points he is advocating a certain worldview. What does his theory lose and gain from relying on psychological arguments? To what degree is it even possible to avoid a dependence upon description?