In my opinion, he describes a basic mechanism of survival for cultures. Cultures that inspire such defensive "inability to think clearly" about the subject have increased survivability (social propagability).

But in order to survive, cultures must also change over time. I see the mechanism for this change as the individual's improvement of the culture for oneself, and then attempting to propagate it to insiders and outsiders. Triggers of this modification include conflicting experience, either direct or indirect via dialogue/argumentation with others.