In the introduction Mill remarks that his object

These are reasonably strong consequentialist arguments againstgiving the state a broad discretionary power to engage in paternalisticlegislation whenever it sees fit. However, they do not support acategorical ban on paternalism. In particular, these arguments provideno principled objection to paternalism—no objection tosuccessful paternalistic restrictions on B's libertythat do in fact benefit B. This weakness in Mill's explicitargument against paternalism is like the weakness in his truth-trackingdefense of freedom of expression. Just as that argument provided noobjection to successful censorship (censorship of all and only falsebelief), so too this argument provides no objection to successfulpaternalism (A's restrictions on B's liberty that dobenefit B). Perhaps some who object to paternalism are onlyconcerned with unsuccessful paternalism. But many would have doubtsabout successful paternalism. For it is common to think thatindividuals have a right to make choices in their own personal affairsand that this includes a right to make choices that are imprudent.

One traditional reconstruction of Mill's proof might look somethinglike this.
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Mill’s enduring interest in the dependencies, evident in was heavily indebted to his earlier absorption in the imperial issues of the 1830s and especially his part in the discussions provoked by the Canadian Rebellion of 1837-38. He was elated in January 1838 by the appointment of Lord Durham as High Commissioner and Governor General of British North America, because this event provided an unparalleled opportunity for the Philosophic Radicals to prescribe for a critical colonial situation. If Durham succeeded, the Radical party no less than the Empire would immediately benefit. Durham took with him to Canada Buller and Wakefield, both of whom substantially contributed to the contents and character of the famous report, including its recommendation for colonial autonomy. Mill for his part promptly employed the to defend Durham and his mission. From this action he derived unusual satisfaction, telling a friend in 1840 “that, as far as such things can ever be said, I saved Lord Durham—as he himself, with much feeling, acknowledged to me. . . .”

Mill comments on the gravity of the issues:

In an early and famous passage Mill conceives of liberalism in termsof “one very simple principle.”
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M. de Tocqueville does not pretend, nor do we, that local self-government should be introduced into Europe in the exact shape in which it exists in New England. An assembly of the rateable inhabitants of a district, to discuss and vote a rate, would usually be attended only by those who had some private interest to serve, and would in general, as is proved by the experience of open vestries, only throw the cloak of democratic forms over a jobbing oligarchy. In a country like America, of high wages and high profits, every citizen can afford to attend to public affairs, as if they were his own; but in England it would be useless calling upon the people themselves to bestow habitually any larger share of attention on municipal management than is implied in the periodical election of a representative body. This privilege has recently been conferred, though in an imperfect shape, upon the inhabitants of all our considerable towns; but the rural districts, where the people are so much more backward, and the system of training so forcibly described by M. de Tocqueville is proportionally more needed,—the rural districts are not yet empowered to elect officers for keeping their own jails and highways in repair: that is still left where the feudal system left it, in the hands of the great proprietors; the tenants at will, so dear to aristocracy, being thought qualified to take a share in no elections save those of the great council of the nation. But some of the greatest political benefits ever acquired by mankind have been the accidental result of arrangements devised for quite different ends; and thus, in the unions of parishes formed under the new poor law, and the boards of guardians chosen by popular election to superintend the management of those unions, we see the commencement of an application of the principle of popular representation, for municipal purposes, to extensive rural districts, and the creation of a machinery which, if found to work well, may easily be extended to all other business for which local representative bodies are requisite.