QUESTION: Rav maintains that Ne'ilah is recited at night, and it replaces the Tefilah of Ma'ariv. The Gemara shows that the Tana of a Beraisa disagrees with Rav and says that Ne'ilah is recited before nightfall and does not replace Ma'ariv. The Gemara defends Rav's opinion and says that the time at which Ne'ilah is recited is the subject of a Machlokes Tana'im, and Rav follows the Tana who rules that Ne'ilah is recited at night.
Which Tana rules that Ne'ilah is recited at night? The Gemara cites a Beraisa in which the Rabanan and Rebbi Yosi disagree about the Tevilah of one who became a Ba'al Keri on Yom Kippur. The Rabanan maintain that he may immerse himself in a Mikvah only until Minchah, but after Minchah has passed he may not immerse. Rebbi Yosi argues that he may immerse at any time during the day. The Gemara understands the argument to be based on whether Ne'ilah may be said at night (after Yom Kippur) or whether it must be said during the day. The Rabanan maintain that Ne'ilah is recited at night (like Rav). Therefore, once a person has recited Minchah there is no need for him to immerse on Yom Kippur in order to pray. Rebbi Yosi says that Ne'ilah must be recited during the day, and therefore even if one has recited Minchah he may still immerse during the day in order to recite Ne'ilah.
The Gemara later cites a different Beraisa which discusses the case of a person who wants to immerse in a Mikvah but he has the name of Hash-m written on his skin. In that case, the Rabanan and Rebbi Yosi argue with regard to a different issue -- whether Tevilah at the prescribed time is a Mitzvah or not ("Tevilah b'Zemanah Mitzvah"). According to Rebbi Yosi, Tevilah b'Zemanah is a Mitzvah, while according to the Rabanan it is not.
If the Rabanan maintain that it is not necessary to perform Tevilah in its proper time, perhaps that is why they say that one may not immerse in a Mikvah during the day on Yom Kippur but must wait until the night (and not because they maintain that Ne'ilah may be recited at night). Perhaps both Rebbi Yosi and the Rabanan agree that Ne'ilah must be recited during the day, and they argue only about a situation in which the person who needs to immerse has already recited Ne'ilah (the Gemara suggests this possibility later). The Rabanan do not allow him to immerse in a Mikvah because he already said Ne'ilah and Tevilah b'Zemanah is not a Mitzvah, while Rebbi Yosi allows him to immerse because Tevilah b'Zemanah is a Mitzvah. Accordingly, there is no Tana who agrees with Rav's ruling that Ne'ilah is recited at night. On what grounds, then, does Rav argue with the Beraisa that says that one may recite Ne'ilah only during the day and that it does not exempt him from Ma'ariv? (TOSFOS YESHANIM DH Michlal)
(a) A simple suggestion would be that the Gemara's conclusion (that the Rabanan maintain Tevilah b'Zemanah Mitzvah) relies on the principle of "Rav Tana Hu u'Palig" -- Rav is a considered a Tana and is entitled to disagree with a Mishnah or Beraisa.
The TOSFOS YOM HA'KIPURIM, however, suggests that it is not necessary to invoke this principle in this case. The MAHARI BEN LEV (Teshuvos 1:13 and 48) infers from the words of TOSFOS in Kidushin (60b, DH Aval) that any Amora may disagree with a Tana -- under one condition. If it is at all possible to interpret the words of another Tana as disagreeing with the first Tana, the Amora may assert that he maintains the issue to be a Machlokes Tana'im. He may rule contrary to the first Tana who clearly contradicts his ruling, even though he cannot conclusively prove that the two Tana'im indeed argue. Accordingly, Rav may assert that he interprets the argument between the Tana Kama and Rebbi Yosi to be whether Ne'ilah may be recited at the night or only during the day (and they discuss a situation in which the person who saw Keri did not yet recite Ne'ilah), and he rules like the Tana Kama who says that it is recited at night.
(b) The TOSFOS YESHANIM and TOSFOS HA'ROSH suggest that Rav may have understood that "Rebbi Yosi" cited in the first Beraisa is not Rebbi Yosi ben Chalafta (the Tana usually referred to as Rebbi Yosi, who maintains that Tevilah b'Zemanah is a Mitzvah), but rather Rebbi Yosi b'Rebbi Yehudah -- who maintains that Tevilah b'Zemanah is not a Mitzvah in another Beraisa cited by the Gemara here. Although the first Beraisa does not clarify which Rebbi Yosi it quotes (which normally is an indication that it is quoting Rebbi Yosi ben Chalafta), nevertheless Rav assumes that it must be referring to Rebbi Yosi b'Rebbi Yehudah, since its wording is so similar to the wording in the second Beraisa which quotes Rebbi Yosi ben Yehudah. Rav did not accept the possibility that the two "Rebbi Yosi"s in the two Beraisos were two different Tana'im.
(c) The TOSFOS HA'ROSH cites the MAHARAM MI'ROTENBURG who suggests that Rav maintained that it is illogical to suggest that the dispute between Rebbi Yosi and the Rabanan involves a person who already recited Ne'ilah. If that would have been the subject of their dispute, the Rabanan would not have said that one may immerse in a Mikvah "until Minchah." They would have said "until Minchah or Ne'ilah, whichever is said later." It must be that they are referring to a person who did not yet recite Ne'ilah, and they do not allow him to immerse on Yom Kippur because Ne'ilah may be said at night. Accordingly, the wording of the Rabanan, "until Minchah," is appropriate: Ne'ilah is not recited during the day and Tevilah is never permitted on Yom Kippur after Minchah.
(It is true that the Gemara suggests at one point that the Beraisa discusses one who already recited Ne'ilah. However, Rav does not accept this possibility as viable unless there is absolutely no other way to understand the Beraisa.)