MEPX
Chapter 3 of 11All chapters
  1. 1How puzzles trick you
  2. 2All, some and none
  3. 3If, then
  4. 4Common fallacies
  5. 5Number sequences
  6. 6Words into equations
  7. 7Rate and work problems
  8. 8Elimination grids
  9. 9Knights, knaves and self reference
  10. 10Reading exactly
  11. 11Building the habit

Chapter 3 of 11

If, then

Rules run one way only.

The valid moves

From if P then Q, finding P gives you Q. Finding that Q is false gives you that P is false. Those two moves are the whole of conditional reasoning.

  • Modus ponens: P is true, so Q is true.
  • Modus tollens: Q is false, so P is false.

The invalid ones

If it rained the street is wet. A wet street does not give you rain, because a burst pipe would do the same job. Reading a rule backwards is called affirming the consequent.