 We have focused so far on understanding Mill’s version of utilitarianism, especially his conceptions of happiness and duty. Now we should consider his justification of utilitarianism, which he offers in his discussion of the “proof” of the principle of utility in Chapter IV. Mill claims that the utilitarian must claim that happiness is the one and only thing desirable in itself (IV 2). He claims that the only proof of desirability is desire and proceeds to argue that happiness is the one and only thing desired. He argues that a person does desire his own happiness for its own sake and that, therefore, happiness as such is desired by and desirable for its own sake for humanity as a whole (“The aggregate of all persons”) (IV 3). He then turns to defend the claim that happiness is the only thing desirable in itself, by arguing that apparent counterexamples (e.g., desires for virtue for its own sake) are not inconsistent with his claim (IV 5–8).