A positivist believes that we can only be certain about things which can be demonstrated through empirical science or mathematics (despite mathematics being the a priori science.) When applied to texts such as the Bible or the US Constitution, positivism would tell us that everything we need to know about those documents is contained within the texts themselves. Most people recognize, if only subconsciously, that positivism is nonsense. Most people also believe that postmodernism, born from the realization that positivism is nonsense, is the polar opposite of positivism. Postmodernism holds that since we cannot be certain about some things such as man's telos, we cannot be certain about anything. Positivism and postmodernism are more like the two sides of one coin. Jacques Derrida and Richard Dawkins have much more in common with each other than either has in common with St. Thomas Aquinas.
Liberalism comes in at this point and proposes itself as the neutral umpire. Anthony Kennedy told us in his wooliest prose that "At the heart of liberty is the right to define one's own concept of existence, of meaning, of the universe, and of the mystery of human life." The liberal argues that since it's impossible for us to be certain, it's unjust to enforce any one narrow vision of the good upon those who might not share that vision. The problem is that it's impossible to avoid this in practice. Government simply is the enforcement of a vision of the good. It's tremendously difficult for liberals to recognize that they're doing this. And since they generally reject metaphysics as a bunch of mystical mumbo jumbo, they have nowhere else to go. Everything becomes mere story-telling - Stephen Colbert's "Truthiness" - and the powerful get to decide what reality is because they can enforce their story.
Because of its incoherence, liberalism appears both mad and hypocritical. Right-liberals often lose the debate as soon as it begins because they've accepted the left-liberal frame: the only alternative to liberalism is obscurantist tyranny. When posed with a false dilemma, the correct solution is to take a third way, which is not simply the golden mean. The middle ground between two lies or between a lie and the truth is still a lie.