There is a land where every inhabitant is either a dayknight or a nightknight. Dayknights tell the truth in the day and lie at night. Nightknights lie in the day and tell the truth at night. An inhabitant says, "Either I am a dayknight or it is day." Is the inhabitant a dayknight or a nightknight, and is it day or night?
There are four choices. Either inhabitant is:
1) A dayknight at day (tells truth).
2) A dayknight at night (tells lie).
3) A nightknight at day (tells lie).
4) A nightknight at night (tells truth).
If the "either __ or __" statement's "or" is exclusive, it works the following way:
Either T or T = F
Either T or F = T
Either F or T = T
Either F or F = F
1) Cannot be the answer since as a dayknight (T) at day (T), statement is false (but dayknight should be truthful).
2) Cannot be the answer since as a dayknight (T) at night (F), statement is true (but dayknight should be lying).
3) Cannot be the answer since as nightknight (F) at day (T), statement is true (but nightknight should be lying).
4) Cannot be the answer since as a nightknight (F) at night (F), statement is false (but nightknight should be truthful).
Given that none can be the answer, we have to assume the "or" is inclusive, so:
Either T or T = T
So, 1) is the solution. Dayknight in the day.

Posted by Monika
on 20120811 20:00:50 