Like Sidgwick, Mill’s On Liberty is on the long side. It’s easy to get lost in those extended paragraphs. This outline is meant to help you navigate your way through it.
The first eight paragraphs identify the problem that On Liberty will address: the tyranny of the majority (¶5). Mill put this problem in historical context by distinguishing it from other questions about political liberty that earlier societies had faced (¶¶2-4). With the expansion of voting rights in England, he worried that the state would be used to enforce conformity. The sociological problem was that people are prone to intolerance. The intellectual problem was that there was “no recognized principle by which the propriety or impropriety of government interference is customarily tested” (¶8).
On Liberty argues for such a principle. It has come to be called 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 …. 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.” (¶9)
The Harm Principle is spelled out in ¶¶9-12. These are the most important paragraphs in this chapter.
The introductory chapter closes with two paragraphs reiterating Mill’s worries about mass intolerance and conformity (¶¶14-15). The last paragraph introduces the topic of the next chapter, liberty of thought and expression (¶16).
Mill will discuss three cases in order to show that there are very great costs to society from suppressing an individual’s opinion. (This is in addition to the indvidual’s costs, of course.)
There are generally three levels of argument for each case. Mill presents his view (first level). Then he considers objections (second level). And then he answers the objections (third level). I won’t go all the way to the third level, but I will note the objections on the second level. The page numbers are to the Hackett edition of On Liberty.
Mill begins by claiming that only those who are infallible could be justified in suppressing an opinion. Since no one is infallible, it follows that suppression is never justified. (p. 17 and throughout)
Mill’s claim: “however true it [an opinion] may be, if it is not fully, frequently, and fearlessly discussed, it will be held as a dead dogma, not a living truth.” There is a contrast here between knowing the truth and believing it without understanding it. (p. 33–34)
Objection: some received opinions are more than half-truths. E.g. the Christian church doesn’t have a partial truth, it has the whole truth. (p. 46) Mill’s answers are on pp. 47-9.
The chapter closes with a summary (p. 50) and some remarks about regulating the manner in which opinions are expressed (pp. 50-2). The summary on p. 50 is especially useful for understanding Mill’s argument in this chapter.