Mill's proof of the principle of utility

Mill's utilitarianism is very closely tied to his instrumentalism; that his argument for the Principle of Utility, while tight, is deeply incoherent; that the incoherence stems from an incoherence in instrumentalism; and that Mill's instrumentalism turns out to have been an island of apriorism in an otherwise empiricist project. It is tempting to think that if Mill had been willing to look to experience at this point also, his theory of practical reasoning, and consequently his moral theory, would have turned out quite differenty"and perhaps less incoherently. So the final moral is that if you're going to be empiricist, be empiricist all the way: about practical reasoning, and about observation.