The fundamental tenet of the liberal conception of free speech is the principle of content neutrality, which Mill espoused in claiming that “there ought to exist the fullest liberty of professing and discussing, as a matter of ethical conviction, any doctrine, however immoral it may be considered.”1. J.S. MILL, ON LIBERTY 36, n. 1 (John Gray and G.W. Smith eds., Routledge 1991) (1859). Note that content-neutrality as a principle of free speech is a different and less ambitious principle from neutrality among conceptions of the good. On this view, the immorality, the falsity, and even the harmfulness of an opinion are not good reasons to censor it. “However positive any one’s persuasion may be, not only of the falsity but of the pernicious consequences” of an opinion, Mill writes, its expression cannot legitimately be prohibited.2. Id. at 43. I am claiming that Mill does not consider harm prevention per se to be even a good (much less a sufficient) reason to censor an opinion or sentiment. This claim is admittedly in tension with some glosses of the so-called harm principle; but, as I have argued elsewhere, the role and content of Mill’s principle of liberty is often fundamentally misconstrued. See Daniel Jacobson, Mill on Liberty, Speech, and the Free Society, 29 PHIL. & PUB. AFF. 276 (2000). Nevertheless, in certain limited circumstances, particular acts of assertion or expression must “lose their immunity” from interference, as Mill puts it, so that we may proscribe incitement to riot, harassment, and the like. Just how to specify those circumstances consistently with content neutrality is a challenge for the liberal program. Of course, this principle and the liberal conception of free speech it embodies are controversial. It can be questioned whether the principle succeeds in protecting the class of speech and other expression that liberals aspire to protect. And even if it does cover the right class, the principle’s justification can be doubted. But I will not discuss these issues, on which there is already an immense literature, any further here.