Freedom of expression, like all widely held political ideals, can be anything from a proud and noble account of some inherent human dignity to a vacuous slogan-idol, and perhaps one so corrupted by the more concrete principles that flesh it out that anything resulting from it can be called freedom of expression only ironically.
Any account of freedom of expression is limited by harmful speech. Minimally, this includes all cases of slander (to harm someone by false speech) and at least some cases of detraction (to harm someone by true speech, like publishing trade secrets). English gives us no general name for the genus “harmful speech”, and so we’re stuck calling it “harmful speech”. Just what gets to count as a case of harmful speech is a problem that can never be resolved by the general principle, and the drama of freedom of expression is resolving the structure it has through the negative space of harmful speech which defines it.
Freedom of expression is allowed when (a.) it impossible to get a consensus over whether X is true or false but (b.) there is a broad consensus that being wrong about X causes no further evils, or at least none worse than an enforced consensus would cause. Historically, one such X was the public expression of the proper confessions of different Christian groups. The idea was that we might never be in a position to get a consensus over the authority of the Pope or the validity of infant baptism, but that belief in this area wouldn’t lead to any further harm, like breaking a man’s leg or picking his pockets, or at least not to a harm as great as enforced consensus. Taken in this sense, freedom of expression is allowed either for various harmless and therefore trivial errors or at least for things that can be trumped by other values, and so freedom of expression applies only to what is trivial or secondary value. On this account we can discern the ultimate values of a group among the topics which are barred from free expression. Thus freedom of expression is not just compatible with the barring persons from holding certain beliefs – whether publicly or privately- it seems to demand that we do it about something or another. We can’t identify what is of secondary value except in light of what is primary.
And so freedom of expression is defined in relation both to harmful speech and to speech about things of ultimate value. Freedom of expression thus concerns expressions about goods of secondary value.
The same sort of reasoning can probably be generalized to other sorts of freedom followed by some objective genitive.