Protagoras was a lawyer, and a teacher. He was a teacher so confident in his own abilities that he used to agree very unorthodox remuneration packages with his students - with one, Euathlus, he agreed that he would receive *no* payment unless the student won his first case. Having completed his training, Euathlus decided that he didn't want the shame of losing a case, but he also didn't want the cost of paying Protagoras: his solution? Don't take on any cases.
Protagoras' was not happy with this turn of events, and decided to sue for his tuition fee. His argument was simple. If Eualthus won, he had won his first case, and must pay Protagoras. If he lost, then he had to pay Protagoras by order of the court. Either way, Protagoras must be paid.
Eualthus, however, had obviously been listening in his lessons. He argued that if he won, he did not have to pay Protagoras, by order of the court. However, if he lost, he clearly would not have to pay by virtue of their agreement.
To me this has never seemed either very interesting or very paradoxical - Eualthus should win, as he hasn't yet won a case. Then Protagoras can sue him *afterwards* for the tuition fee (as Eualthus will now have won his first case). However, if we are to take the paradox at face value, it is simply another liar paradox, which we will meet again, and discuss at greater length at that juncture.