Appeal To Widespread Belief (Bandwagon Argument, Peer Pressure, Appeal to Common Practice):the claim, as evidence for an idea, that many people believe it, or used to believe it, or do it.If the discussion is about social conventions, such as "good manners", then this is a reasonable line of argument.However, in the 1800's there was a widespread belief that bloodletting cured sickness. All of these people were not just wrong, but horribly wrong, because in fact it made people sicker. Clearly, the popularity of an idea is no guarantee that it's right.Similarly, a common justification for bribery is that "Everybody does it". And in the past, this was a justification for slavery.
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