It is argued that the moral theory undergirding J.S. Mill's argument in On Liberty is a species of perfectionism rather than any kind of utilitarianism. The conception of human flourishing that it invokes is one in which the goods of personal autonomy and individuality are central. If this conception is to be more than the expression of a particular cultural ideal it needs the support of an empirically plausible view of human nature and a defensible interpretation of history. Neither of these can be found in Mill. Six traditional criticisms of Mill''s argument are assessed. It is concluded that in addition to depending on implausible claims about human nature and history Mill''s conception of the good contains disabling incommensurabilities. It is argued that these difficulties and incommensurabilities plague later liberal thinkers such as Isaiah Berlin and Joseph Raz who have sought to ground liberalism in a value-pluralist ethical theory. No thinker in Mill''s liberal posterity has been able to demonstrate the universal authority of liberal ideals
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