In the Introductory section of his great work On Liberty Mill states clearly the limits to state power over the liberty of the individual:
The object of this Essay is to assert one very simple principle, as entitled to govern absolutely the dealings of society with the individual in the way of compulsion and control, whether the means used be physical force in the form of legal penalties, or the moral coercion of public opinion. That principle is, 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. … In the part which merely concerns himself, his independence is, of right, absolute. Over himself, over his own body and mind, the individual is sovereign.
About this Quotation:
2009 was the 150th anniversary of the first publication of J.S. Mill’s On Liberty (1859). Mill makes it very clear from line one of the book what his concerns are, namely to explore the consequences of the one central idea which he thinks should predominate in politics - “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”. This should be compared to the opening of The Subjection of Women (1869) where he states concisely a similar governing principle for the book: “That the principle which regulates the existing social relations between the two sexes—the legal subordination of one sex to the other—is wrong in itself, and now one of the chief hindrances to human improvement; and that it ought to be replaced by a principle of perfect equality, admitting no power or privilege on the one side, nor disability on the other.” It took an editor after Mill’s death to publish a volume (1879) which combined these two complementary works. We have this volume online at the OLL.