A man who had mastered Scripture, studied the Mishnah, and served many scholars dropped dead in the middle of his life. His widow seized his tefillin (leather phylacteries worn during prayer) and carried them from synagogue to synagogue, weeping. "Is it not written, That is thy life, and the length of thy days (Deuteronomy 30:20)? My husband did everything right. Why did he die young?"
No one could answer her. Then Elijah, of blessed memory, appeared. "My daughter, what was your husband's conduct during the days of your impurity?" She bristled. "God forbid! He would not touch me even with his little finger. He would say, 'Touch no vessel lest you bring me into a state of doubt.'" Elijah pressed further. "And in the later days, after the flow had ceased?" She hesitated. "We ate together. We slept in one bed. Our bodies touched. But he had no other intention."
Elijah's verdict was devastating: "Blessed be the All-Present for slaying him. The Torah declares, Thou shalt not approach unto a woman as long as she is impure (Leviticus 18:19). Even skin touching skin, even with pure intentions, violates the boundary God drew around intimacy."
From this the rabbis expanded the principle of fences. The Torah forbids approaching a forbidden relation (Leviticus 18:6). The sages extended it: a man must not be alone with any woman at an inn, even his sister. He must not walk behind a woman in the marketplace. He must not engage in frivolous conversation. Every boundary existed to prevent a person from drifting, step by imperceptible step, toward transgression.
"Thy belly is like a heap of wheat set about with lilies" (Song of Songs 7:3), the rabbis taught. Even a pile of wheat left unguarded will be eaten. The lilies are the fences. And the School of Hillel added: teach everyone, not only the worthy, because even sinners drawn near to Torah study have produced righteous and pious generations.