Edgar Abercombie visited an island which had the following features:
- Every inhabitant was a knight who always tells the truth, or a knave who always lies.
- Every inhabitant was sane and had only correct beliefs, or was mad and had only false beliefs
- When asked a yes/no question, they will simply show you a red card or a black card. One card signifies yes, and the other no, and this is dependent on the person.
Is it possible to ask a single yes/no question, to determine if there is gold on the island?
This problem is taken from Raymond M. Smullyan's book Logical Labyrinths.