The Riddle of Scheherazade by Raymond M. Smullyan

Author:Raymond M. Smullyan [Smullyan, Raymond]
Language: eng
Format: epub
ISBN: 978-0-307-81983-3
Publisher: Knopf Doubleday Publishing Group
Published: 1997-05-19T16:00:00+00:00

CHAPTER XX

Which Personality?

Jack and John are twin brothers and both have dual personalities. Jack in his normal state always tells the truth, but in his altered state always lies. John is the opposite; in his normal state, John always lies, but in his altered state, he always tells the truth. The two are indistinguishable in appearance; their mother is about the only person who can tell them apart. Several interesting problems arise:

1

Suppose you meet one of the brothers one day and wish to know whether he is Jack or John. You may ask him only one question answerable by yes or no, and the question must be simple, not compound—that is, it must not contain logical connectives such as and, or, not, if/then. There is a perfectly simple, straightforward question that will do the job. What question will work?

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