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?