Edgar Abercrombie was an anthropologist who was particularly interested in the logic and sociology of lying and truth-telling. One day, he visited the island of Knights (who always tell the truth) and Knaves (who always lie), and met three people, Andrew, Bernard and Charlie.

Andrew: "Exactly one of us is a knave."

Bernard: "Exactly two of us are knaves."

Charlie: "All of us are knaves."

Which of them are knaves?

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How many knights are there?

If there were 0 knights, then Charlie would be telling the truth and this would lead to a contradiction. Therefore, there is at least 1 knight. Since Andrew, Bernard and Charlie are saying different things about themselves, this means that then two are knaves and one is a knight. This means that Bernard is the knight, and Andrew and Charlie are knaves.