Gemara

Question (Rav Chisda to Rabah): You must agree that Chachamim can uproot a Torah law!

(Beraisa) Question: If a man was married (mid'Rabanan) to a minor, from what point does he inherit her and become Tamei to engage in her burial (when she dies), and permit her to eat Terumah (if he is a Kohen)?

Answer #1 (Beis Shamai): It is from when she becomes a Na'arah (and has Chupah);

Answer #2 (Beis Hillel): It is after Chupah;

Answer #3 (R. Eliezer): It is after (she grows up and) they have Bi'ah.

(A Kohen is permitted and commanded to become Tamei only for his seven close relatives.) The Beraisa says that her husband mid'Rabanan becomes Tamei to bury her. Mid'Oraisa, her father becomes Tamei for her!

Answer: She is like a Mes Mitzvah (any Kohen may bury an unattended corpse).

Objection: She is not a Mes Mitzvah!

(Beraisa): A Mes Mitzvah is one that has no one to bury it. If he calls and others answer, this is not a Mes Mitzvah.

Answer: Here, since her relatives do not inherit her, if she calls, no one will answer (they will not want to bury her).

Mo'ed Katan 20b (Beraisa): One is Mis'avel over all relatives listed in Parashas Kohanim for whom a Kohen becomes Tamei: one's wife, father...

Rishonim

Rif and Rosh (Mo'ed Katan 12b and 3:35): One is Mis'avel over all relatives listed in Vayikra for whom a Kohen becomes Tamei: one's father... and wife.

Rambam (Hilchos Evel 2:1): Mi'Divrei Sofrim a husband is Mis'avel over his wife if he made Nisu'in, and she is Mis'avel over him.

Question (Kesef Mishneh): The Rambam (Halachah 7 and 3:1) says that one may become Tamei for his wife. Surely, this is mid'Oraisa! If so, also Aveilus is mid'Oraisa!

Answer #1 (Radvaz): The Rambam's text in Mo'ed Katan did not list Ishto among the relatives for whom one is Mis'avel mid'Oraisa, rather, among those whom were added mid'Rabanan. The Sifra includes Ishto from "li'Sh'ero ha'Karov Elav". The Rambam explains that it is a mere Asmachta.

Note: The Ritva (Mo'ed Katan 20b) asks why the Gemara there does not list 'Ishto'. Rashi (Shabbos 105b) does not include 'Ishto' among those whom the Torah requires one to bury, but Rashi (Vayikra 21:2) says that li'Sh'ero refers to Ishto.

Mizrachi (on Rashi Vayikra 21:2 DH Ein): Perhaps the Rambam's text included Ishto among those who were added mid'Rabanan. It is better to say that she was not listed because an Asmachta includes her.

Question (Lechem Mishneh): "Lo Yitama Ba'al b'Amav Lehechalo" forbids a Kohen to become Tamei for a forbidden wife. This clearly shows that the Torah permits him to become Tamei for a permitted wife! It seems that the Rambam was forced to say that it is only mid'Rabanan because a husband is not Mis'avel with his wife over all her relatives. The Rambam rules that inheritance of a wife is only mid'Rabanan, and this is also expounded from li'Sh'ero. "Lo Yitama Ba'al b'Amav" need not refer to a husband (the Targum explains that it refers to a great man).

Answer #2 (Kesef Mishneh): The Rambam calls anything not explicit in the Torah 'mi'Divrei Sofrim', even if it is mid'Oraisa. It is not obvious that "li'Sh'ero" refers to Ishto. The Targum of li'Sh'ero is 'to his relative'!

Rambam (7): A Kohen must become Tamei for his wife b'Al Korcho (against his will). This is only mi'Divrei Sofrim. Chachamim made her like a Mes Mitzvah. Since he is the sole heir, no one else would bury her.

Question (Ramban, ha'Ramach, Ba'al ha'Hashlamah, Kesef Mishneh): Yevamos 89b says that Chachamim made her like a Mes Mitzvah only regarding a minor married mid'Rabanan!

Answer #1 (Kesef Mishneh): Indeed, Li'Sh'ero permits an adult married mid'Oraisa. Since also an adult is like a Mes Mitzvah (he is the sole heir, so no one else would bury her), the Rambam gave a reason that applies to all wives.

Answer #2 (Beis Yosef YD 398 DH u'Mah she'Chosav b'Shem): The Rambam means that to Metamei for any wife is mi'Divrei Sofrim, i.e. not explicit in the Torah. Regarding a minor he says that she is like a Mes Mitzvah.

Answer #3 (Lechem Mishneh 2:1): Mar Ukva intended to be Misavel with his wife (Mo'ed Katan 20b). He holds that Aveilus over one's wife is mid'Oraisa. The Rambam can say that the Sugya in Yevamos is like Mar Ukva.

Mishneh l'Melech: Perhaps even after Chachamim considered her like a Mes Mitzvah and commanded her husband to become Tamei for her, her father is still allowed to become Tamei for her. However, if so why did the Gemara say 'mid'Oraisa, her father becomes Tamei for her'? Perhaps it stresses that even though he is commanded to, since he does not inherit her he will not.

Rambam (3:1): If a Kohen becomes Tamei for anyone except for one of his six relatives listed in the Torah or for his wife, he is lashed.

Rosh (Hilchos Tum'ah (after Menachos) Siman 2): Our text connotes that if the Mes would be alive and call, his relatives would come to bury him. If his relatives would not come, he is a Mes Mitzvah, even if other Yisraelim would bury him. The Rambam says that her family would not bury her since their inheritance passes to the husband unlike Torah law, and all the more so strangers would not bury a Mes with heirs and relatives. Since her burial is uncertain, Chachamim imposed it on her husband, so there will not be arguments about it. Kavod ha'Briyos overrides a Lav in such cases. The Yerushalmi explains that 'he calls and others answer' refers to if the Kohen who finds the Mes. Our text was corrupted by scribes who had not learned the Yerushalmi.

Nimukei Yosef (28b DH uvi'Gmara): Some say that a Mes Mitzvah is when no one answers when he calls, i.e. the person about to die has no relatives. Even if there are others who would bury him, he is a Mes Mitzvah. Even a Kohen or Nazir may bury him, like it says here, since her relatives do not inherit her they would not answer her, so she is a Mes Mitzvah. This is wrong. A Kohen may become Tamei for a Mes Mitzvah due to Kavod ha'Briyos (to avoid disgrace), so the Mes will not remain unburied. If others could bury it, there is no disgrace! The Yerushalmi says that 'he calls no one answers' refers to the Kohen who finds the Mes; therefore, he may become Tamei. Even if he could hire people to bury it, he may choose to become Tamei. The Gemara exaggerates when it says that even if she would call, her relatives would not answer her, all the more so strangers. Normally Tum'as Kohen for his relatives is not due to Mes Mitzvah; relatives and strangers would bury them. Rather, the Torah wanted that there be people readily available to bury them.

Tosfos (Yevamos 89b DH Kivan): Because she resembles a Mes Mitzvah, it does not appear like Chachamim uproot the Torah.

Question (Heichal Yitzchak EH 2:73:40): The Yerushalmi (Gitin Perek 5) says that Chachamim have the power to uproot the Torah. What is the source for Chachamim's power? Why is the Bavli unsure? Also, there are good reasons to rely on one witness for Edus Ishah, but considering a wife to be a Mes Mitzvah is flimsy!

Answer #1 (Heichal Yitzchak 41): Chachamim have the power to consider their enactments mid'Oraisa. They were reluctant to do so b'Kusim Aseh, lest this cause people to belittle Divrei Torah. This concern does not apply to a Hora'as Sha'ah, or when there is some reason.

Answer #2 (heard in the name of Rav Moshe Shternbuch): Surely Chachamim have the power to uproot the Torah. The question is whether or not they ever used it.

Teshuvas ha'Rashba (3:252): The Heter to become Tamei does not depend on inheritance. The Torah allows a Kohen to become Tamei "l'Achoso ha'Besulah", even though he inherits her even if she was widowed or divorced from Nisu'in. Only when someone does not inherit mid'Oraisa and Chachamim enacted that he inherits, then the mid'Oraisa heirs would refuse to inherit and she is like a Mes Mitzvah. This applies to a minor married mid'Rabanan, and to a Yavam, for he inherits whatever sell to the Yevamah in her husband's lifetime.

Poskim

Shulchan Aruch (YD 373:4): A Kohen becomes Tamei for his wife ha'Nesu' who is permitted to him, but not for his sister ha'Arusah, even if she is Arusah to a Kohen.

Pischei Teshuvah (3): The Mishneh l'Melech (Evel 2:7) says that a Kohen can be Metamei for his mid'Rabanan wife only because she is like a Mes Mitzvah (Yevamos 89b). If his sister was Mekudeshes mid'Rabanan this does not apply, so the Mitzvah mid'Oraisa of Tum'as Kerovim is not uprooted. This is wrong. We know that Chachamim can tell people to refrain from Mitzvos. The concept of Mes Mitzvah was needed to justify becoming Tamei through an action.

Shulchan Aruch (YD 399:13 and OC 548:3): If a Mes died and was buried on Yom Tov Sheni of Chutz la'Aretz at the end of the Regel, the first day of Aveilus, which is mid'Oraisa, overrides Yom Tov Sheni, which is mid'Rabanan. This is for the seven relatives for whom the Torah permits Tum'as Kohanim.

Gra (YD 15): This is unlike the Rambam, who says that Aveilus for one's wife is mid'Rabanan.