Fallacies are not simply crazy ideas. They are usually both plausible and logical — but with something missing.
Sometimes what is missing in a fallacy is simply a definition. Undefined words have a special power in politics, particularly when they engage people’s emotions. “Fair” is one [such word]. While the fact that the word is undefined is an intellectual handicap, it is a huge political advantage. People with very different views on substantive issues can be unified and mobilised behind a word that papers over their differing, and sometimes even mutually contradictory, ideas. Who, after all, is in favour of unfairness? Similarly with “social justice”, “equality”, and other undefined terms that can mean wholly different things to different individuals and groups.
Thomas Sowell, Economic Facts and Fallacies