Paul Graham is somewhat a victim of this himself.
For example, the assumption behind this very essay is that there is such a thing as a rigid, singular concept of "truth" in the moral, cultural and political sphere, that there is a "fact of the matter" whether a belief (say one PG holds) is correct or not. Or that that we live in a static world where such truths can even exist, and not, in a fluid, dynamic, politically messy world where contrasting viewpoints interact and produce something not something ever lasting, but something which is fragile and must always be fought for, this fight being a necessary feature for a functioning democracy.