"The only proof capable of being given that an object is visible, is that people actually see it. The only proof that a sound is audible, is that people hear it: and so of the other sources of our experience. In like manner ... the sole evidence it is possible to produce that anything is desirable, is that people do actually desire it. ... No reason can be given why the general happiness is desirable, except that each person, so far as he believes it to be attainable, desires his own happiness. ... [E]ach person's happiness is a good to that person, and the general happiness, therefore, a good to the aggregate of all persons. Happiness has made out its title as one of the ends of conduct, and consequently, one of the criteria of morality. (IP3 585bf)

"But it has not, by this alone, proved itself to be the sole criterion. To do that, it would [be] necessary to show, not only that people desire happiness, but that they never desire anything else." (IP3 586a)