MEPX
Chapter 7 of 10All chapters
  1. 1What an argument is
  2. 2Validity and soundness
  3. 3If, then
  4. 4All, some, none
  5. 5Necessary and sufficient
  6. 6Deduction against induction
  7. 7Formal fallacies
  8. 8Informal fallacies
  9. 9Logic puzzles
  10. 10Using it in real arguments

Chapter 7 of 10

Formal fallacies

Invalid shapes that feel valid.

The main pair

Affirming the consequent reads a conditional backwards. Denying the antecedent assumes that removing one cause removes the effect. Both survive because they sound reasonable.

  • Test by inventing a case where premises hold and the conclusion fails.
  • If you can build that case, the argument is invalid whatever the topic.

Undistributed middle

All cats are mammals, all dogs are mammals, therefore cats are dogs. Sharing a category does not connect two groups, though the pattern is common in political argument.