The Mishna (2a) listed the 15 women who – in the event that their husband passes on with no offspring – cannot become a yevama and marry his surviving brother, due to the fact that they are closely related to him (e.g. if the brother who passed away had married his niece, her father cannot perform yibum with her). Our Gemara presents a question posed by Levi (ben Sisi) to Rebbi (Rabbi Yehuda HaNasi), who asks why the Mishna only lists 15 women, rather than 16. Rabbi Yehuda HaNasi immediately recognized that the suggestion Levi was making aimed to include the case of anusat aviv – when a man rapes a woman, who becomes pregnant with a son. If the rapist has a son from another marriage, according to the hakhamim that son is permitted to marry the woman who was raped (even though he would not be allowed to marry a woman who was legally married to his father – see Vayikra 18:8). If he does so, and dies without children, we could have a situation where the surviving brother’s own mother could potentially be his yevama!
Rabbi Yehuda HaNasi’s reaction to this suggestion is a blunt ke-medumeh li she-ein lo mo’ah be-kodkodo – “it appears to me that this man does not have a brain in his head!” He explains that this unusual case is subject to a difference of opinion between Rabbi Yehudah and the hakhamim, and the Mishna would not feel a need to include it.
The questioner, Levi ben Sisi, lived in the generation between the tanna’im and amora’im, and was a disciple of Rabbi Yehuda HaNasi who redacted the Mishna. He was also close personal friends with him and with his son, Rabbi Shimon. Nevertheless, it is not uncommon to find (as we do in this story) Rabbi Yehuda HaNasi rebuking him in strong terms. Still we find statements in the Talmud that point to the great respect that Rabbi Yehuda HaNasi had for his student. After Rabbi Yehuda HaNasi passed away, Levi could not find an appropriate teacher, so he moved to Bavel, settling in Neharde’a and developing a close relationship with Shmuel’s father and with Rav.
As was the case with several of Rabbi Yehuda HaNasi’s students, Levi edited an important collection of baraitot, known as Matnita d’bei Levi.