Protagoras was a Greek sophist (a class of men in ancient Athens who taught philosophy, dialect, rhetoric etc. for money) who lived in the fifth century B.C.
It is said that Protagoras once agreed with one of his pupils that he could delay payment for his tuition until he had won his first case.
The agreement seemed sound, until the student was sufficiently qualified to start offering his services professionally. He hung a sign outside his office, but no business came his way.
Frustrated, Protagoras decided to sue his former pupil.
Can you see the paradox that subsequently arose?
Protagoras reasoned that he was on to a "sure thing". He reasoned that either he would win the case, in which case his pupil would be required by the courts to pay for his tuition, or he would lose, in which case his former pupil would have won his first case and would therefore be required to pay for his tuition under the terms of their original agrreement.
His former pupil disagreed. He reasoned that either he (the former pupil) would lose that case, in which case he would still never have won a case and would therefore not be required to pay under the terms of their original agreement, or he would win the case and the court would rule that he did not have to pay.
Who was right?