Yevamos 12a: Tzaras Mema'enes and the Mechanics of Miyun

The shiur opens with three core difficulties in the Gemara (גמרא)'s discussion of tzaras mema'enes. First, Tosfos asks: if miyun retroactively nullifies marriage, why should tzaras mema'enes be prohibited at all—she should be like any tzarah of an ervah who never entered kedusha. Second, there is a shakla v'tarya about whether the case is ba'al or yavam, and whether geirushin after miyun creates a new problem. Third, why does the Gemara delay its question from the Mishna—the Mishna says miyun during the yavam's lifetime permits the tzarah, so why didn't the Gemara immediately ask how tzaras mema'enes could ever be prohibited? Rabbi Zweig explains that the Rambam (רמב"ם) (Hilchos Gerushin 11:17) rules that a woman who performs miyun on a yavam is permitted to all the yavam's relatives except his father (due to the gezeirah of kalaso). The Maggid Mishneh explains this is because miyun operates retroactively (nekra lemafreia), erasing the original marriage as if she were never married at all. The Rambam further rules that if she performs miyun on one brother (Shimon), she is permitted to marry another brother (Levi). The Maggid Mishneh argues that this second ruling is the proof for the first: the only way Levi could marry her is if she is now a "stranger" (nochris) to the family, which demonstrates miyun undoes kedusha retroactively.