Amongst ways to justify freedom of speech, the metaphor of the marketplace of ideas has often held a central place. The basic thought is simple enough: just as free markets are likely to contribute the most to social and economic progress, so a free market in ideas is most likely to allow us to discover truth.
You might question whether this analogy really works (what’s the “price” of an idea? etc.). But quite independent from these worries, I think that most people who use this metaphor fail to explore what it would fully entail.
Here’s a first problem: a marketplace for ideas would be a truth-conducive market only if there was demand for truth, or at least if truth was the primary feature in ideas that individuals demanded. It’s not at all clear that this is the case. People also demand ideas which comfort them, which rationalise their identity, their grievances, and their self-interest.
Marketplaces, if they work, deliver on what people want. So there’s no guarantee that a working marketplace of ideas is particularly good on delivering the truth. Indeed, that publications like the Daily Mail or Fox News are successful might be a sign that the marketplace of ideas is healthy and thriving.
That’s of course not a knock-down argument. Insofar as some demand is driven by a desire for truth, free competition between ideas in this area might still be the instrumentally best way to achieve truth. (Perhaps you are very optimistic and think this is the case for academia.) But our defence of a free marketplace for ideas has now become importantly conditional: it is under certain conditions and in certain respects that it delivers epistemically valuable results; but where the conditions do not hold, we might have no reason to favour a free marketplace over some other kind of organisation.
The second crucial point is that the analogy cannot at all establish unrestricted freedom of speech. Outside libertarian circles, few people will believe that markets should be totally unregulated. Indeed, a standard liberal (“ordoliberal”) idea is that the state should step in where markets fail. Governments should, for example, reign in monopolies, force companies to internalise external costs, and protect vulnerable market participants from exploitation. Markets in modern economies are free as much as they’re regulated, and most liberals think that this is as it should be.
So there’s no reason to think that we should not also regulate the marketplace of ideas. I’m not sure what precise regulations on free speech we should favour. But invoking the metaphor of the marketplace works very badly if you wish to argue for unrestricted freedom of speech, and in fact may show the precise opposite.
One might reply that there are no natural analogues in the speech case for the reasons why we wish to regulate markets in goods and services—what, for example, could it possibly mean that an idea has external costs? But once you accept that there are few meaningful analogies between real-world markets and the “market” of ideas, it would be better just to drop the analogy altogether.