Paul Graham writes a rather long-winded essay on how the definition of morals, taboos and heresies shift from time and place. Criticizing nuclear weapon stockpiling in 1950s USA would have you branded as “unamerican” or a “communist”. Preaching religious tolerance in Europe during the Crusades could get you imprisoned. Both were unpopular opinions in their times, but now are accepted as moral truths.
In essence, he hits the nail on the head on why critical thinking is important. Catastrophic events such as the Spanish Inquisition or the Holocaust occur because no one questioned the common beliefs at the time. Innocent folks died because not enough people were able to realize, and say, “Hey, something’s not right here.”
Graham offers some suggestions. “Instead of being part of the mob, stand as far away from it as you can and watch what it’s doing. And pay especially close attention whenever an idea is being suppressed.”
Be wary of labels. “If a statement is false, that’s the worst thing you can say about it. You don’t need to say that it’s heretical. And if it isn’t false, it shouldn’t be suppressed. So when you see statements being attacked as x-ist or y-ic (substitute your current values of x and y), whether in 1630 or 2030, that’s a sure sign that something is wrong.”
It doesn’t mean you’re right and they’re wrong, but you have to reach that conclusion independently. Or else you end up being a lemming.