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.