Mill says in chapter 1 of On Liberty that the object of the essay is to assert one very simple principle: “that the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is self-protection. That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others” (CW XVIII, 223 [1, 9]). A person's own good, if of adult age and in a civilized community, “is not a sufficient warrant. He cannot rightfully be compelled to do or forbear because it will be better for him to do so, because it will make him happier, because, in the opinions of others, to do so would be wise or even right” (CW XVIII, 223–4 [1, 9]).
Mill says that the basis for his argument for this principle is utility: “I forgo any advantage which could be derived to my argument from the idea of abstract right, as a thing independent of utility. I regard utility as the ultimate appeal on all ethical questions; but it must be utility in the largest sense, grounded on the permanent interests of man as a progressive being” (CW XVIII, 224 [1, 11]).
This essay is addressed to the way in which Mill makes his case for liberty on the basis of utility “in the largest sense, grounded on the permanent interests of man as a progressive being.”