“The best lack all conviction, while the worst are full of passionate intensity.” – William Butler Yeats, “The Second Coming”
Ignorant people are more confident in their limited knowledge than people who are actually knowledgeable. In other words, the more you know the more you realize how little you know.
This is actually, measurably and measured, true, according to Sam Harris in The Moral Landscape. Which explains how reasonableness – and actual experts and scientists – struggle so much in the face of creationists, climate change deniers, and those who somehow think ending the Bush tax cuts for the wealthy will boost the economy.