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Knights and Knaves
An if-then is a promise. It's only broken when you do your part and they don't do theirs.
- Propositions, truth tables, and the one row that breaks p → q (vacuous truth)
- Converse vs. contrapositive — and the Wason card test (A and 7, not A and 4)
- Negating "for every": the opposite of everybody's failing is somebody isn't
- Proof by cases: knights and knaves without knowing everything
edit_note Activity · Knights and knaves, by cases
- A says "B is a knave." B says "A and C are the same type." Write down both possibilities for A.
- Case 1, A is a knight: A tells the truth → B is a knave → B's sentence is false → A and C differ → C is a knave.
- Case 2, A is a knave: A lied → B is a knight → B's sentence is true → A and C are the same → C is a knave.
- Every case says C is a knave, so C is a knave. ∎ (You never needed to know A.)
Journal this
THE PROOF FRAMEWORK GIVEN: what do we know? (write the definitions out) PROVE: exactly what must be shown — for every case STRATEGY: direct · contrapositive · contradiction · cases · induction DERIVATION: every line + its reason ∎ did we prove the original claim, every case?
Talk about it
- Treating p → q as broken when p is false (it's vacuously true)
- Mixing up the converse q → p with the contrapositive ¬q → ¬p
- Negating "everybody fails" as "everybody passes" (it's "somebody doesn't fail")
- Stopping after one case of a proof by cases





