John Stuart Mill
 On Liberty
One of Mill's most influential works is "On Liberty," where he spells out his grand thesis The Harm Principle:
“”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 war-ranted, 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 civilised community, against his will, is to prevent harm to others. His own good, either physical or moral, 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. These are good reasons for remonstrating with him, or reasoning with him, or persuading him, or entreating him, but not for compelling him, or visiting him with any evil in case he do otherwise. To justify that, the conduct from which it is desired to deter him must be calculated to produce evil to some one else. The only part of the conduct of any one, for which he is amenable to society, is that which concerns 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.
As a fun aside, it seems Thomas Jefferson beat him to the punch by almost 100 years:
“”The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbour to say there are twenty gods, or no god. It neither picks my pocket nor breaks my leg.
This needs clarification. Unfortunately, to the untrained eye, this reeks of modern libertarianism. It is not. Notice that Mill phrases it four different ways in his thesis: "is self-protection", "is to prevent harm to others", "deter... evil to some one else", "is that which concerns others". The last is considerably more open-ended. The libertarian distinction of positive vs negative liberties is entirely alien to Mill, and The Harm Principle did not intend to deal with only one of the two varieties. Modern libertarians, due to their peculiar lens of positive and negative liberties which is absent from Mill's work, commonly misunderstand and misconstrue Mill and The Harm Principle. If any doubt remains, read the following passage from On Liberty:
“”There are also many positive acts for the benefit of others, which he may rightfully be compelled to perform; such as to give evidence in a court of justice; to bear his fair share in the common defence, or in any other joint work necessary to the interest of the society of which he enjoys the protection; and to perform certain acts of individual beneficence, such as saving a fellow creature’s life, or interposing to protect the defenceless against illusage, things which whenever it is obviously a man’s duty to do, he may rightfully be made responsible to society for not doing.
 On non-governmental moral majority tyrannies
Moreso than most philosophers, Mill was particularly concerned about the collective effect of actions of individual members of society to punish or curtail behavior. Examples might include shunning, McCarthy-like blacklists, etc. He considered such societal pressure to be capable of being as powerful or more powerful than a formal governmental policy. In his view, this kind of pressure can also abridge someone's right of self determination, just like a formal governmental policy. To make this point, Mill distinguishes between "remonstrating [...] reasoning [...] persuading [and] entreating" vs "compelling," claiming the first kind is acceptable behavior without legitimate reasons, and the second kind — force itself — requires legitimate reasons, reasons which concern the well-being of other people.
 On conservatism
|—Letter to the Conservative MP, Sir John Pakington (March 1866)|
Mill, of his own free will, on half a pint of shandy was particularly ill.