James is doing a wonderful job explicating J.S. Mill's defense of free speech in On Liberty. He's now turned to what I think is one of the best of Mill's arguments - that free speech is required because, as a student just said, "no one has a monopoly on the truth." Given human fallibility, we need free speech to discover the truth. Competition among various views of the world will generate a better approximation to truth than would happen if any limits were imposed upon speech. Thus our ignorance becomes the justification for freedom, as freedom allows us to generate knowledge that would otherwise go undiscovered. Mill is very clear on this point.
Of course this argument is precisely parallel to the Hayekian/Austrian argument about the way in which the market works as a discovery process. (See Hayek's essay "Competition as a Discovery Procedure.") What I find especially interesting about this is that the Hayek essay was written in the late 1960s (although embryonic versions of the ideas were around earlier), after Hayek had been engaged in his fairly deep study of Mill. Hayek long admired Mill and edited a collection of Mill's letters with Harriet Taylor. In the early 50s, he and his second wife took a European vacation that retraced some steps that Mill had taken.
Given Hayek's admiration for Mill, I wonder how much of what emerged in the 1960s in Hayek's thinking about competition came from his study of Mill and Mill's argument for free speech in On Liberty? Furthermore, some contemporary Austrians, myself included, have argued that markets are extra-linguistic communication processes, so one way of seeing the parallels in Mill and Hayek is to argue that all forms of communication should be free because all forms of communication, whether speech or markets in this case, are ultimately discovery processes that are socially necessary to overcome our structural ignorance.
I think this same argument can be extended to Darwinian evolution as well. Evolution via natural selection is a very similar sort of discovery process as markets and free speech. The implied vision of human natural and social life as being an interconnected set of evolutionary discovery processes is, for me, quite inspiring. We are all connected in our biological, social, economic, and intellectual evolution by similar sorts of discovery processes.