The Paradox of the Court is a logic problem from ancient Greece. Protogoras took on a pupil, Euathlus, on the understanding that Euathlus would pay him after he won his first court case. After Protogoras taught him the law, Euathlus decided not to practice, and Protogoras sued him for the amount owed.

Protagoras argued that if he won this lawsuit, he’d be paid the money he was owed, and if Euathlus won the suit, then he’d have won his first case and would owe Protagoras the money anyway under the terms of their contract. So he ought to be paid either way.

Euathlus argued that if he won the suit then by the court’s decision he owed nothing, and if he lost the suit then he still would not have won his first case, and thus owed Protagoras nothing under the contract.

One lawyer suggested that the court should decide in favor of the student and declare that he doesn’t have to pay for his education. Then Protagoras should sue him a second time — since then, incontrovertibly, the student will have won his first case!