John Stuart Mill: '"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. In like manner, I apprehend, 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 . . . . [W]e have not only all the proof which the case admits of, but all which it is possible to require, that happiness is a good: that each person's happiness is a good to that person, and the general happiness, therefore, a good to the aggregate of all persons."'