The famous sophist Protagoras took on a promising pupil, Euathlus, on the understanding that the student pay Protagoras for his instruction after he wins his first court case. After instruction, Euathlus decided to not enter the profession of law, but to enter politics instead, and so Protagoras decides to sue Euathlus for the amount he is owed. There are other theories though. I hope this helps!