After explaining how popular opinions might be false, Mill makes three further arguments in favor of freedom of opinion.
His second argument (after the argument discussed last section that the popular opinion could be false), is that even if the popular opinion is true, if it is not debated it will become "dead dogma." If truth is simply held as a prejudice, then people will not fully understand it, and will not understand how to refute objections to it. Dissent, even if it is false, keeps alive the truth against which it dissents.
Mill then turns to two potential criticisms of his argument.
First, one could say that people should be taught the grounds for their opinions, and that having been taught these grounds, they do not then merely hold prejudices but really understand the basis of their opinions. Mill replies that in cases where differing opinions are possible, understanding the truth requires dispelling arguments to the contrary. If a person cannot refute objections, then he cannot properly be said to understand his own opinion. Furthermore, he must hear these objections from people who actually believe them, because it is only these people who can show the full force of the arguments. Responding to objections is so important that if no dissenters exist, it is necessary to imagine them, and to come up with the most persuasive arguments that they could make.
A second criticism might be that it is not necessary for mankind in general to be familiar with potential objections to their beliefs, but only for philosophers or theologians to be thus aware. Mill replies that this objection does not weaken his argument for free discussion, because dissenters still must be given a voice with which to object to opinions. Furthermore, while in the Catholic Church there is a clear distinction between common people and intellectuals, in Protestant countries like England, every person is considered responsible for his choices. Also, in modern times it is practically impossible to keep writings that are accessible to the intellectuals from the common people.
Mill then presents a third argument for the value of liberty of thought and discussion. He writes that if a true opinion is not debated, the meaning of the opinion itself may be lost. This can be seen in the history of ethical and religious beliefs--when they stop being challenged, they lose their "living power." Mill says that Christianity faces such a situation, where people's beliefs are not reflected in their conduct. As a result, people do not truly understand the doctrines they hold dear, and their misunderstanding leads to serious mistakes.
Mill presents one possible criticism of this view. He writes that it could be asked whether it is essential for "true knowledge" for some people to hold erroneous opinions. Mill replies that having an increasing number of uncontested opinions is both "inevitable and indispensable" in the process of human improvement. However, this does not mean that the loss of debate is not a drawback, and he encourages teachers to try to compensate for the loss of dissent.
Mill then turns to a fourth argument for freedom of opinion. He writes that in the case of conflicting doctrines, perhaps the most common case is that instead of one being true and one false, the truth is somewhere between them. Progress usually only substitutes one partial truth for another, the newer truth more suited to the needs of the times. Dissenting or heretical opinions often reflect the partial truths not recognized in popular opinion, and are valuable for bringing attention to a "fragment of wisdom." This fact can be seen in politics, where differing opinions keep both sides reasonable. In any open question, the side that is least popular at the time is the side that should be most encouraged. This side reflects interests that are being neglected.
Mill then looks at a criticism of this fourth argument. He says that it could be argued that some principles, such as those of Christianity, are the whole truth, and if somebody disagrees, he is completely wrong. Mill replies by saying that in many ways Christian morality is "incomplete and one-sided," and that some of the most important ethical ideas have been derived from Greek and Roman sources. He argues that Christ himself intended his message to be incomplete, and that it is a mistake to reject secular supplements to Christian morality. Most basically, human imperfection implies that a diversity of opinion would be required to understand truth.
After looking at these four arguments for liberty, Mill briefly addresses the argument that free expression should be allowed, but only if it sticks to "fair discussion." He says that such a standard would be very hard to enforce from a practical perspective. Mill posits that it would likely only be dissenters who would be held to such a high standard of conduct. Ultimately, it is not law's place to restrict discussion in this way; public opinion must look at individual cases, and hold both sides to the same standard.
Mill makes the case that if people hold a true opinion they will benefit from hearing dissenters argue against that opinion. He also observes that he thinks most people only know partial truths, and that they might benefit from hearing other fragments of truth. This discussion reflects a particular conception of how people learn. Mill contends that people learn through debate, and through having their opinions challenged. Thus, dissenting opinions are socially useful because they help people to understand the real strength (and limitations) of their own beliefs. Mill believes that the usefulness of dissenting opinions cannot be substituted for, neither when the unpopular view is partially true, nor when it is completely false.
One idea to consider when thinking about Mill's argument is whether he has an overly idealized view of this learning process. For example, what happens when the conflicting opinions rest on fundamentally different presuppositions--are the conversations that Mill describes really possible? If people do not share the same vocabulary for discussing moral and political issues, then will they really be challenging each other, or simply talking past each other? Think about what answer Mill might give to this problem. If his answer is unconvincing, then can he still say that a diversity of opinions is socially useful?
Finally, it is also worth looking at Mill's refutation of someone who thinks that Christianity is the whole truth. Mill seems to argue that such a person misinterprets Christianity. Would this response be convincing to a person with views on Christianity that are different from Mill's? Does Mill have other arguments that might provide a better response to this claim? More generally, Mill's discussion of religious toleration in Chapter 2 brings up the issue of whether Mill can be convincing to people whose beliefs demand intolerance of those who disagree with them. Since Mill is using social benefit as the basis of his justification for liberty, it would seem that a person who believes in intolerance could simply say that any benefits of free opinion are outweighed by allowing something evil to be expressed. Think about how persuasive such a critique is, given Mill's claims about the need for dissent in order to truly understand one's own opinions.