A strange episode is preserved in the Talmud: a witch once transformed a man into an ass. He found himself in the marketplace on four legs, mounted like any beast of burden. One of her companions, more honest than her friend, broke the spell and the ass became a man again. The medieval commentator Rashi remarked that the man whose name appears in the story, Yannai, "cannot have been a rabbi of any standing, for he practiced witchcraft." But later authorities corrected Rashi — the name is preserved in Sofrim chapter 16.
Why did the sages think such things possible? Because they read the world through a famous teaching (Sanhedrin 67b): "Ten measures of witchcraft came into the world. Egypt received nine, and all the rest of the world one."
Egypt, in the rabbinic imagination, was the laboratory of every dark art. Moses had to contend not only with Pharaoh's armies but with the full arsenal of Egyptian sorcery (Exodus 7:11). The plagues were not tricks against superstition; they were blows against the one place on earth where sorcery had taken up permanent residence.
Even long after the Exodus, the sages warned, Egypt's nine measures remained in the world's bloodstream. A Jew might be changed into a beast at a street corner — and only God's Name, spoken plainly, could change him back.