The people on the Island of Knights and Knaves are either knights or knaves. Knights are honest and virtuous, and whatever they say is true.
Knaves are base and shameful, and whatever they say is false.
Stranger encounters two islanders, A and B, who are having a quarrel, accusing each other of being a knave. Solution One must be a knight and the other a knave. But since the information is completely symmetric between the two of them the stranger cannot determine who is what. Reasoning If they were both knaves they would both be speaking the truth something knaves would never do. If they were both knight they would both be lying something knights would never do.