A very special island is inhabited only by knights and knaves. Knights always tell the truth, and knaves always lie. You meet 6 inhabitants: Grace, Michael, Noah, Henry, Oliver, and David. As Grace put it, "Oliver is not a knave". Michael remarked, "Grace is a knave or Oliver is a knight". In a statement by Noah: "David is a knave or Grace is a knave". "Grace is a knave if and only if Michael is a knave" - Henry. Oliver noted, "Noah is a knave or Michael is a knight". David stated, "Grace is a knave and Noah is a knave". So who is a knight and who is a knave?
Grace is a knight, Michael is a knight, Noah is a knight, Henry is a knight, Oliver is a knight, and David is a knave.
(1) Grace is a knight
(2) Michael is a knight
(3) Noah is a knight
(4) Henry is a knight
(5) Oliver is a knight
(6) David is a knave
Assume Grace is a knight. No contradiction is found in their claim that Oliver is not a knave. Assume Oliver is a knight. No contradiction is found in their claim that Noah is a knave or Michael is a knight. Assume Noah is a knight. No contradiction is found in their claim that David is a knave or Grace is a knave. David cannot be a knight, because this would contradict the claim of Noah that David is a knave or Grace is a knave. Assume David is a knave. No contradiction is found in their false claim that Grace is a knave and Noah is a knave. Assume Michael is a knight. No contradiction is found in their claim that Grace is a knave or Oliver is a knight. Assume Henry is a knight. No contradiction is found in their claim that Grace is a knave if and only if Michael is a knave.