Common Errors
in Logic and Rhetoric
©2000 Edward G. Rozycki
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edited 3/31/01
Errors In The Structure Of Argument
1. Undistributed Middle:
a. Apples are fruits. Oranges are fruits. So apples are oranges.
b. Sam reads Karl Marx, so he must be a communist because Communists read Karl Marx.
2. Unwarranted Converse:
a. All dogs are warm-blooded. Your pet is warm-blooded. So it must be a dog.
b. John must be a businessman, since businessmen support immigration and so does John.
3. Some to All
a. Some animals are meat-eaters. Your pet is an animal. So it must be a meat-eater.
b. Some businessmen support unrestricted immigration. John is a businessman, so he must support unrestricted immigration.
4. Negating the Antecedent
a. All oranges are fruit. Since this is not an orange, it is not a fruit.
b. Since union members oppose unrestricted immigration and John is not a union member, he does not oppose unrestricted immigration.
Rhetorical Errors
5. Ad Hominem
a. "You have to be crazy or perverted to believe that just because all dogs are mammals, my dog is a mammal!
b. John supports unrestricted immigration but he's a jerk!
6. Irrelevant Authority
a. You can be confident that advertising tells the truth because nine out of ten TV celebrities engage in making ads.
b. "Liberalism destroys!" my uncle Rush, the famous statistician, always says.
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