MEPX
Chapter 9 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 9 of 11

Knights, knaves and self reference

Statements about the speaker.

Test each assumption

In puzzles where some people always lie and others always tell the truth, assume the speaker is truthful, follow it through, and see whether anything contradicts. Then assume the opposite.

  • A statement that is impossible for a truth teller must come from a liar.
  • "I am lying" cannot be said consistently by either, which is the paradox these puzzles play with.

Questions that work either way

The classic solution to the two guards problem is a question whose answer is the same from a liar and a truth teller, by asking what the other one would say.