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.