Ascribed to the sophist philosopher Protagoras (c.490-420 BC).
A lawyer teaches law to a student without fee on condition that the student will pay him when he qualifies and wins his first case.
However, when the student qualifies he takes up another profession. The lawyer sues him for his fees, on the grounds that if he wins, he is paid and if he loses, the student has won and so must pay by the agreement.
The student is unperturbed because if he wins he need not pay the fees, and if he loses he does not owe them. There is some confusion concerning the agreement here, but logical rules preventing the application of a condition to itself certainly resolve the paradox.