The Mishnah (8:3) rules that male Ammonites and Moabites (and their descendants for eternity) are prohibited from marrying into the Jewish community. Ammonite and Moabite females are excluded from this prohibition (provided that they convert).5
The exclusion of the females was subject to debate (Gemara Yevamot 76b). It is an important debate touching on the validity of the Davidic line of kings (which will include the Mashiach). David HaMelech was the great grandson of the famous Moabite convert – Rut. David’s son Shlomo HaMelech married Naamah, an Ammonite convert, and their son Rechavam continued the Davidic line. How could such a fundamental Halacha be subject to debate?
Just before Pesukai D’Zimrah in Shacharit, the siddur lists the thirteen hermeneutic principles through which the Torah is elucidated (e.g. Kal Vachomer). The Great Sanhedrin would use these principles to, among other things, derive from the text of the Torah details of Halachot that were previously unknown or forgotten. Details that were derived in this manner could be overruled by any later Great Sanhedrin if they determined that a different derivation was more valid (Rambam, Hilchot Mamrim 2:1).6
On the other hand, if the Halacha in question was given directly to Moshe by Hashem (a ‘Halacha LeMoshe MiSinai’) it could never be overruled. A Halacha would only be classified as a Halacha LeMoshe MiSinai if there was a reliable tradition to that effect.7 A case in point is towards the end of the third Mishnah. After arguing with the Chachamim about the status of female Egyptians and Edomites,8 R' Shimon concludes that the arguments are irrelevant because he has a tradition from his teachers that his opinion is a Halacha LeMoshe MiSinai.
The Brisker Rav uses this principle to explain a difficulty in Megillat Rut.9 Before Boaz married Rut he offered Rut’s closest relative the opportunity to marry her. The relative refused on the basis that he might ruin his inheritance (according to Rashi ‘inheritance’ means ‘future children’). He feared the biblical prohibition against marrying a Moabite. Boaz, who was head of the Great Sanhedrin, assured him that the prohibition only applied to male Moabites. However the relative still refused to marry Rut.
The Brisker Rav asks, if Boaz declared in the name of the Great Sanhedrin that only male Moabites were prohibited, how could the relative dismiss this? Also, why was the relative only concerned about the effect on his children? If the relative held that the marriage to a female Moabite was prohibited, he should have refused on that basis.
The Brisker Rav explains that the relative was not concerned about personally transgressing. He could rely on the interpretation of the Great Sanhedrin and marry Rut. His concern was that a future Great Sanhedrin might overrule this interpretation and decide that female Moabites were also prohibited. This would mean that his children would be tainted as descendants of a Moabite and would be prohibited from marrying into the Jewish community.
In fact, the Great Sanhedrin’s interpretation was challenged three generations later (Yevamot 76b-77a). Sha’ul HaMelech’s adviser, Doeg the Edomite, challenged David’s fitness for kingship on the basis that he was a descendent of the Moabite Rut and therefore forbidden to marry into the Jewish community. The Great Sanhedrin of the day argued that the prohibition only applied to male Moabites, however Doeg was able to refute all of their arguments. Yeser the Israelite came to the rescue and saved the Davidic line of kings; he testified that he had a tradition from his teachers that the exclusion of female Moabites from the prohibition was a Halacha LeMoshe MiSinai and was therefore not subject to challenge.
However a difficulty still remains with this episode in Megillat Rut. Surely Boaz would have explained to Rut’s relative that the Halacha relating to female Moabites was a Halacha LeMoshe MiSinai and was therefore not subject to challenge. Why then did the relative continue to refuse to marry Rut? Even though he knew that the Halacha in question was a Halacha LeMoshe MiSinai, it seems that he was afraid that a future Great Sanhedrin might forget this fact and try to overrule this Halacha. The incident with Doeg the Edomite and David proves that this was a valid concern.
5 Ammonites and Moabites are not identifiable today because the Assyrian King Sennacherib exiled them and moved them to a different location. Therefore this prohibition no longer applies.
6 In contrast, Rabbinical enactments and minhagim could only be overruled by a Great Sanhedrin that was superior in wisdom and in number. Gezayrot (precautionary measures to ensure biblical prohibitions would not be transgressed) could never be rescinded by any Great Sanhedrin. (Hilchot Mahmrim 2:2-3).
7 This was before the oral law was written down.
8 The dispute is set out in the Gemara (Yevamot 77b).
9 Quoted in ‘The Dynamics of Dispute’ by Rabbi Zvi Lampel.
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