In Chapter 2, Mill turns to the issue of whether people, either through their government or on their own, should be allowed to coerce or limit anyone else's expression of opinion. Mill emphatically says that such actions are illegitimate. Even if only one person held a particular opinion, mankind would not be justified in silencing him. Silencing these opinions, Mill says, is wrong because it robs "the human race, posterity as well as the existing generation." In particular, it robs those who disagree with these silenced opinions.
Mill then turns to the reasons why humanity is hurt by silencing opinions. His first argument is that the suppressed opinion may be true. He writes that since human beings are not infallible, they have no authority to decide an issue for all people, and to keep others from coming up with their own judgments. Mill asserts that the reason why liberty of opinion is so often in danger is that in practice people tend to be confident in their own rightness, and excluding that, in the infallibility of the world they come in contact with. Mill contends that such confidence is not justified, and that all people are hurt by silencing potentially true ideas.
After presenting his first argument, Mill looks at possible criticisms of his reasoning and responds to them.
First, there is the criticism that even though people may be wrong, they still have a duty to act on their "conscientious conviction." When people are sure that they are right, they would be cowardly not to act on that belief and to allow doctrines to be expressed that they believe will hurt mankind. To this, Mill replies that the only way that a person can be confident that he is right is if there is complete liberty to contradict and disprove his beliefs. Humans have the capacity to correct their mistakes, but only through experience and discussion. Human judgment is valuable only in so far as people remain open to criticism. Thus, the only time a person can be sure he is right is if he is constantly open to differing opinions; there must be a standing invitation to try to disprove his beliefs.
Second, there is the criticism that governments have a duty to uphold certain beliefs that are important to the well being of society. Only "bad" men would try to undermine these beliefs. Mill replies that this argument still relies on an assumption of infallibility—the usefulness of an opinion is still something up for debate, and it still requires discussion. Furthermore, the truth of a belief is integral to whether it is desirable for it to be believed.
Mill observes that the assumption of infallibility about a certain question implies that one not only feels very sure about a belief, but also includes the attempt to try to decide that question for other people. It is in stifling dissenting opinions in the name of social good that some of the most horrible mistakes in human history have been made. Mill writes about Socrates and Jesus Christ, two illustrious figures in history, who were put to death for blasphemy because their beliefs were radical for their times. Mill then considers whether society should be able to censor an opinion that rejects a common moral belief or the existence of God and a future state. He gives the example of Emperor Marcus Aurelius, a just and kind man who still persecuted Christianity, failing to see its value to society. Mill argues that if one is to accept the legitimacy of punishing irreligious opinions, one must also accept that if one felt, like Marcus Aurelius did, that Christianity was dangerous, one would also be justified in punishing Christianity.
Third, Mill considers the criticism that truth may be justifiably persecuted, because persecution is something that truth should have to face, and it will always survive. Mill replies that such a sentiment is harshly unfair to those who actually are persecuted for holding true ideas. By discovering something true, these people have performed a great service to humanity. Supporting the persecution of such people suggests that their contributions are not truly being valued. Mill also contends that it is wrong to assume that "truth always triumphs over persecution." It may take centuries for truth to reemerge after it is suppressed. For example, Mill writes that the Reformation of the Catholic Church was put down twenty times before Martin Luther was successful. It is mere sentimentality to think that truth is stronger than error, although truth will tend to be rediscovered over time if it is extinguished.
Fourth, Mill responds to the possible argument against him that since we do not actually put dissenters to death any more, no true opinion will ever be extinguished. Mill replies that legal persecution for opinions is still significant in society, for example in the case of blasphemy or atheism. There is also no guarantee, given general public opinion, that more extreme forms of legal persecution will not reemerge. In addition, there continues to be social intolerance of dissent. Mill argues that societal intolerance causes people to hide their views, and stifles intellectualism and independent thought. Stifling free thinking hurts truth, no matter whether a particular instance of free thinking leads to false conclusions.
In Chapter 2, Mill looks exclusively at issues of freedom of thought and of opinion. It is significant that he attempts to justify the importance of this freedom by showing its social benefits—for Mill, diversity of opinion is a positive societal good.
Mill's argument that the dissenting opinion may be true brings up some important points. First, it highlights that Mill believes that moral truths do exist. Thus, in defending liberty, Mill does not say that all opinions are equally valid. Mill is not a relativist; he is not saying that all things can be true according to their circumstances. Rather, he is simply saying that any single idea might be true, and that for this reason no idea can be dismissed, since truth is a boon to progress.
Second, Mill tries to show the contingency of popular beliefs about truth while going to great lengths to not actually state that any popular views about things like religion are wrong. To accomplish this, he observes that in the past people have been persecuted for what is now believed to be true. Thus, Mill creates a logical situation in which anyone reading must accept that if they support persecuting "false" views, then they are required to accept their own persecution if in the minority on a specific issue. Mill is thereby able to dismiss the persecution of "false" views, without condemning modern views as being false.
Third, Mill's examples of persecuted truths reflect some of his rhetorical strategies in this essay. Mill is very conscious of his audience in 19th century England, and he uses examples, like the crucifixion of Christ, which would certainly have resonance with his readers. This reflects a more general strategy in this essay of choosing familiar and often uncontroversial examples in order to make much broader moral claims. In reading this essay it is important to remember that England did not have the same legal protection of liberty that it has today; Mill uses examples to make his points that would not get him into trouble with the law or English society.
Finally, it is worth thinking about the importance of Mill's assumption in the existence of truth to his justification for freedom of opinion. If no one could be wrong or right, would this require tolerance and respect of difference, or could the strongest opinion simply try to defeat all others? Mill does not try to answer this question, because the existence of truth is assumed throughout. However, thinking about such issues is important in seeing how persuasive Mill can be to people who do not share all of his assumptions.