Lewis does three things:
So what’s neturalism? A neutralist insists on arguments for tolerance that can be ‘sold on both sides of the street’. That is, they insist that a sound argument for tolerance has to be such that it could be addressed to all potentially intolerant parties. If it works, everyone is tolerant for the same reasons.
What’s wrong with it? It won’t explain why anyone should be tolerant when it comes to their strongest beliefs. The only person for whom it might have some appeal is Caspar Milquetoast, the man who would rather avoid an argument than stand up for his deepest convictions.
Is Mill really a neutralist? He seems to be one insofar as his argument for tolerance is that it will promote something that everyone (presumably) favors: truth. Everyone wants to know the truth, even if it means finding out that they're wrong. Right? Well, that's Mill's assumption; compare Hobbes 11.21. Leaving that aside, I wasn’t sure that Mill always toed the neutralist line.
Lewis’s explanation of how a society might come to value tolerance, and why it makes sense for its members to continue doing so, reminds me quite a bit of Hobbes. Lewis has the advantage of doing without Hobbes’s sophisticated theory of rights. That’s an advantage because it means he isn’t committed to explaining the relevance of his theory without an actual contract.
Keep Lewis’s rationale for tolerance in mind for when we get to Rawls’s Political Liberalism. Lewis’s argument is a version of what Rawls will call a “mere modus vivendi”. Rawls thinks a mere modus vivendi is inadequate and we’ll want to ask why he thinks that and whether he’s right.