Rabbi Yehudah HaLevi (c. 1075 to 1141), the great Hebrew poet and physician of medieval Spain, author of the philosophical work The Kuzari, was urged by his wife to find a match for their daughter. Yehudah, distracted by a poem in progress, answered rashly. I will give her in marriage to the first Jew who knocks at our door. And he swore it as an oath. His wife, wiser than her husband in that moment, said nothing.
The next morning a knock came. The door opened. A young man in rags stood on the threshold, a traveler with road dust still on his face, poor and unremarkable. Yehudah's stomach dropped. An oath was an oath. He welcomed the stranger in.
Unable simply to hand his daughter over to a man he had never seen before, Yehudah offered the young man a compromise. Stay in my house as my pupil, he said. Study Torah under me. Make yourself worthy of my daughter, and then she will be yours. The young man agreed and set to work.
His progress was astonishing. Within weeks he was answering questions at the level of a rabbi. Yehudah marveled but said nothing. Then one evening Yehudah was at his writing table, wrestling with a particularly difficult stanza in a new piyyut, a liturgical poem. He could not complete the final strophe. He threw down his pen and went to dinner late and distracted. The young man, passing the open study door afterward, glanced at the unfinished page and, without any effort at all, wrote in the missing line. When Yehudah returned and read what had been written, he knew who his student was.
The ragged traveler was Rabbi Abraham ibn Ezra (c. 1089 to 1167), the wandering genius, biblical commentator, poet, astronomer, and grammarian, one of the most brilliant Jewish minds of the twelfth century. He had disguised himself so as not to be recognized and to be judged not by his reputation but by the quality of what he actually offered. He became Yehudah HaLevi's son-in-law that same week. This exemplum, preserved as number 356 in Moses Gaster's 1924 Exempla of the Rabbis, teaches that oaths have a way of delivering exactly the blessing you would have refused, if you had known who was knocking.