MEPX
Chapter 3 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 3 of 10

If, then

The two valid moves and two invalid ones.

Valid

From if P then Q: given P, conclude Q. Given not Q, conclude not P. Those two, modus ponens and modus tollens, are the whole toolkit for conditionals.

  • Modus tollens is the logic of falsification in science.
  • The contrapositive of a conditional is always equivalent to it.

Invalid

Given Q, concluding P is affirming the consequent. Given not P, concluding not Q is denying the antecedent. Both are extremely common and both are wrong.