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Knights and Knaves Puzzle

Knights and Knaves puzzles all fall into the category of logic puzzles and can easily solved by making the right deductions.


What is a Knights and Knaves Puzzle

All the puzzles in this category take place on an island where the inhabitants are either knights who always tell the truth or knaves who only lie. The puzzles revolve around one visitor to the island who meets a small group of locals and has to determine whether their knights or knaves. In some cases, the visitor can ask a local directly what he is or he can use simple Yes/No questions.


How to Solve a Knights and Knaves Puzzle

There are many types of puzzles in this category, so let’s focus on one of the most popular. Let’s say the visitor meets three inhabitants of the island named A, B and C. He asks the first one, A, what he is, but unfortunately does not hear the answer so he has to rely on the other two locals. B says A’s answer was ‘I’m a knave’, while C jumps in ‘Don’t believe him, B is a liar’. Who’s telling the truth and who’s what?

If you’re at all familiar with logic you’ll notice that what B says cannot possibly be true. A couldn’t have said ‘I’m a knave’ as this falls under the liar’s paradox. If a liar says he’s a liar than he’s telling the truth, which is impossible.

Therefore, it is easy for the visitor to assume that A said ‘I’m a knight’ (which might be or not be true), B is definitely a liar, while C is certainly telling the truth. As far as A is concerned there is not enough information to determine whether he’s a liar or not.


Knights and Knaves Puzzle Versions

There are many variations to this popular puzzle. For instance the visitor must determine the identity of a local by using one question. It cannot be a direct question like ‘Are you a knave’ as both the knight and the liar would say No. The trick is to use a question for which the answer is already known. One solution for this puzzle would be asking the local something like ‘Are you a tree frog?’ The knight would obviously say No, while the liar would be forced to say Yes, which the visitor knows it’s a lie.