Mill's free speech doctrine is distinct from, yet compatible with, his central principle of ‘purely self-regarding’ liberty. Using the crucial analogy with trade, I claim that he defends a broad laissez-faire policy for expression, even though expression is ‘social’ or other-regarding conduct and thus legitimately subject to social regulation. An expedient laissez-faire policy admits of exceptions because speakers can sometimes cause such severe damage to others that coercive interference with the speech is justified. In those relatively few contexts where interference is called for, however, the central principle of self-regarding liberty sets absolute limits to the scope of society's regulatory authority. Regulation can never amount to an outright ban of any type of expression that can be consumed by the individual without direct and immediate harm to others. Nevertheless, and perhaps surprisingly, the central liberty principle admits censorship of certain extraordinary types of expression which necessarily harm others.