Honestly, I haven't read much of his work, but I've came across that quote before and I felt it really described the importance of fighting intolerance and not letting fascists, homeophobes, bigots, etc. spread their hate speech without challenging them. I'll have to read that book you mentioned now, besides I need a break from Das Kapital, it's giving me headache. Hopefully I can find his book online somewhere. Thanks for recommending it.
You quoted Popper! Kudos bro! Have you read the "Open Society and Its Enemies?"
