Isn't prejudice just common sense? If it's reasonable to assume that since every

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Isn't prejudice just common sense? If it's reasonable to assume that since every dog I've ever met has four paws and a snout, then, until proven otherwise, every dog I will meet with have four paws and a snout, then why isn't it reasonable to assume, if every American I've ever met is foolish, unless proven otherwise, that every American I will meet will be foolish?