Where do we draw the line between criticism and bigotry? From the uproar over Lee Anderson’s remarks about the London mayor, Sadiq Khan, being “controlled” by Islamists to the condemnation of slogans used on pro-Palestinian demonstrations, it is a question at the heart of current debates about Muslims and Jews, Islam and Israel. The distinction between criticism and bigotry should, in principle, be easy to mark. Discussions about ideas or social practices or public policy should be as unfettered as possible. But when disdain for ideas or policies or practices become transposed into prejudices about people, a red line is...