Mill must be making the assumption that, in general at least, believing the truth is a way of achieving happiness.

If that is so, what harm can be done by suppressing a false view? In fact, there are very strong reasons against doing so, Mill argues, even if we could know it to be false. If we do not consider challenges to our opinion, then ‘however true it may be, if it is not fully, frequently, and fearlessly discussed, it will be held as a dead dogma, not a living truth’ (On Liberty, 161). As Mill says, we ‘go to sleep at [the] post as soon as there is no enemy in the field’ (On Liberty, 170). One danger here is that the real meaning of the view might be lost or enfeebled if it is not constantly challenged and defended, and so becomes ‘deprived of its vital effect on the character and conduct, the dogma becoming a mere formal profession, inefficacious for good’ (On Liberty, 181). But perhaps the great danger is that when challenged by a sparkling presentation of the opposite, false, view, the champions of the received truth will be unable to defend themselves. Not only will they look foolish, but the false view may gain a popularity it does not merit, sometimes with disastrous consequences.

This, according to some accounts, is what has happened to evolutionary theory in the United States. Believers in Darwinism, while realizing the theory has some apparent flaws, nevertheless did not take seriously the thought that any intelligent, scientifically trained person could fail to accept the broad truth of evolutionary theory in some form or other. Consequently, when well-organized and skilful religious fundamentalists started packaging and deliberately mixing up sophisticated and plausible objections to Darwinism with their own advocacy of ‘creation science’—the literal belief in the Old Testament—the Darwinian establishment was not ready to meet the challenge. And so the creationists developed a following way out of proportion to the scientific merits (nil) of their theory. Many Americans—in certain southern states a majority—still believe that evolutionary theory should not be taught in schools.

(Jonathan Wolff, An Introduction to Political Philosophy, rev. ed. [New York: Oxford University Press, 2006], 110-1 [brackets in original])

Note from KBJ: Wolff, an Englishman, misrepresents the American debate. Nobody wants evolutionary theory banished from the schools. What many people want is inclusion of other theories, such as Design Theory, or at least a discussion of criticism (some of it by reputable scientists) of evolutionary theory. The dogmatists in this debate are the Darwinians. They should take Mill’s argument to heart: If they think alternative theories are false or inferior, they should argue as much. If they think the criticisms of evolutionary theory are confused or weak, they should argue as much. If truth is on their side, and if they’re half as smart as they think they are, they should have no trouble establishing the superiority of their theory. As for what should be taught in public schools, that’s not a scientific or a philosophical question; it’s a question of public policy, concerning which scientists and philosophers, as such, have no expertise.