Received wisdom is what “everybody knows” is true without anyone having to think about it. Received wisdom has a lot of defense mechanisms: for example, trading in unexamined assumptions, avoiding contrary evidence, dismissing the need of evidence at all, or demonizing those who question it. Question-begging slogans are another. Mantras like “nothing to do with Islam” or “war on women” substitute for evidence and analysis. Another is “you can’t say that,” used to dismiss or marginalize comments or proposed policies by assuming disastrous consequences, or implying that saying such things is morally repugnant and, to quote Obama’s favorite obfuscation, “doesn’t represent who we are as a country.” In fact, “you can’t say that” is usually an ideological weapon, or an excuse for inaction.