The Mekhilta DeRabbi Yishmael draws a legal ruling from God's command to the Israelites before the revelation at Sinai: "Do not draw near to a woman" (Exodus 19:15). Moses delivered this instruction three days before the Torah was given at Mount Sinai, requiring the Israelites to abstain from marital relations in preparation for the encounter with the Divine Presence.
From this three-day period of separation, Rabbi Elazar ben Azaryah derived a specific ruling about ritual purity: if a woman ejects semen on the third day after intercourse, she is considered ritually pure. The logic works backward from Sinai. God told Moses to instruct the people to separate for three days. This implied that after three days, any trace of the prior intimacy would no longer render a person impure. If three days were sufficient preparation for standing before God at Sinai, then three days must be the legal threshold for this particular form of purification.
This method of deriving law from narrative is classic Mekhilta. The Sinai revelation was not merely a historical event. It was a legal precedent. Every detail of how Israel prepared for receiving the Torah became a source of binding halakhah (Jewish religious law). The three-day separation established a measurable standard that the rabbis could apply to ongoing questions of ritual purity.
Rabbi Elazar ben Azaryah's ruling demonstrates how the rabbis treated the Sinai narrative as simultaneously history and legislation. The command "Do not draw near to a woman" was not just practical advice for a specific occasion. It was divine instruction that established a permanent legal principle, one that would be debated and refined in the Talmud (Shabbat 86a) for centuries to come.