Rava says there is a ribuy from the word "kol" in the pasuk of "kol aveidas achicha" that is michayev you to save someone's land from getting damaged, not just mitaltilin (like shor, seh, simlah, etc.).
R'Chiya tries to support Rava's opinion from a Beraisa that teaches that you have to fence off a person's land from imminent flooding, where it could get damaged.
Rava is docheh the raya by saying that it could be the Beraisa is not talking about THE LAND you have to save from getting damaged, rather, you're saving THE WHEAT that's attached to the land from getting damaged - and the chiddush is that the wheat still has a din of mitaltalin (and you have to save it) even though it's attached to the land and you might think you wouldn't have to save it (because there is no source for having to save karka, if Rava is incorrect.)
In the next part of the Gemara, though, Rava seems to have a good raya to himself from the way he explains a diyuk in the Mishna.
According to Rava, the diyuk case in the reisha of the Mishnah, of roeh BEIN HAKERAMIM, that WOULD be an aveidah, is specifically referring to a situation where the cow itself is not in any danger, but it's going to damage THE LAND, and when the Mishnah says you're chayiv, it's m'shum aveidas KARKA (not gufa of the animal).
Why doesn't Rava (or R'Chiya) support the ribuy of "kol" that includes a chiyuv to even save karka, from the circumstances of this diyuk case in the reisha of the Mishnah?
(Granted, ABAYE learns there is no diyuk to be made - he learns running is ALWAYS aveidas gufa, even in a kerem, but Rava in the Gemara clearly shows how the Mishnah cannot support his explanation, m'svara.)
Daniel Steinberg, Columbus, OH USA
1) Daniel, before I answer your question I want to point out that it is not Rebbi Chiya who tries to support Rava's opinion. Rebbi Chiya is a very early Amora (in fact, the Gemara on 5a states that Rebbi Chiya is a Tana who is capable of disagreeing with a Mishnah), while Rava is a late Amora who was born long after Rebbi Chiya died.
According to our text, it is Rav Chananyah who tried to support Rava. However, Rabeinu Chananel here writes that it is Rabah speaking here, not Rava. The Poras Yosef here (printed in the back of the Gemara) also points out that this might fit well with the Gemara above on 6b, where it seems that Rav Chananyah lived at the same time as Rabah (but this is also later than Rebbi Chiya).
2) Now to the question itself: I found your question asked by the author of "Even ha'To'ein" (available on Otzar ha'Chochmah) on Bava Metzia 31a, page 678, #6. He writes that Rava indeed believed that one must say that there is a proof to his opinion from the Mishnah. However, Rav Chanaya could not cite a support from the Mishnah because Rava himself requires support for the way he is learning the Mishnah. Since Abaye does not agree with this way of learning the Mishnah, it follows that support from the Mishnah would not be a strong enough way of backing up Rava.
3) I think we can compare this to what the Gemara sometimes says (see Sukah 26a and Gitin 28b), "Arvecha Arva Tzarich" -- "your guarantor requires a guarantor." One borrows money and the creditor demands that a guarantor should sign that, in case of default, he will pay up instead. But if the guarantor himself is not sufficiently reliable, one would have to find a second guarantor for the first guarantor. That means that the first guarantor is really not worth much. The parable is that if the support that Rav Chananyah would cite from the Mishnah is itself in need of support, then we have not achieved very much. This is why Rav Chananyah offered support from a Beraisa, for which we do not yet know of a way to refute.
4) Thinking again about this, it seems to me that I have not explained things sufficiently.
a) Again, before we start, let us first get clear who the sages in this Sugya are. Our Gemara states, "Amer Lei Rav Chananyah l'Rava Tanya d'Mesayei'a Lach," and this bears a remarkable similarity to the Gemara above on 6b: "Amer Lei Rav Chananyah l'Rabah Tanya d'Mesayei'a Lach."
The only difference is that in our Gemara it reads "Rava" while on 6b it reads "Rabah," but we have already noted that the text of Rabeinu Chananel on 31a is "Rabah" and this also the text of the Rosh on 31a.
Therefore, from now on, I think that I will refer to the two Amora'im in our sugya as Rabah and Rav Chananyah.
b) It should be pointed that it would not at all be the way of the Gemara for Rav Chananyah to say to Rabah that the Mishnah is a support for his opinion. This is because everyone knew the Mishnah, and if it would be explicit in the Mishnah that one must prevent land from being damaged, then Rabah would not need to say this because the Amora'im do not need to state Halachos which have already been taught in the Mishnah, since everyone knows these. The Amora'im do not tell us Halachos that everyone knows; they teach us novel ideas that one cannot see explicitly in the Mishnah.
c) In contrast, it is appropriate that Rav Chananyah should point out to Rabah that there is support from the Beraisa for what he said, because while the Amora'im were expected to know the entire Mishnah, they were not expected to know every Beraisa.
d) Again, the din of preventing land from being damaged is hinted at in the Mishnah, according to Rabah's opinion, but that hint would not be a sufficiently strong proof for Rabah against Abaye. The Gemara (Taanis 21a) tells us that Ilfa said that every Din in the Beraisos of Rebbi Chiya and Rebbi Oshiya has a source in the Mishnah. There are many Halachos that are hinted at in the Mishnah, but Rav Chananyah found a Beraisa which said almost the identical words as the Din of Rabah, so it was more useful to mention this support.
B'Hatzlachah Rabah,
Dovid Bloom
Thank you, R'Dovid, as usual for taking the time to look up the answers and providing your thoughtful, and insightful answers.
My mistake - I was writing my question from memory about who the Amora was that was providing a support from the Beraisa to Rava (Rabah), and named R'Chiya instead of Rav Chananya. Thanks for clarifying that.
In terms of the answer, if I understand correctly, you're saying that:
The Mishnah would not have been brought as a siyua to Rabah anyways, because Amoraim are understood to only be stating chiddushim that are not found in Mishnayos.
Even if the above wasn't an issue - this particular Mishnah would not have been brought as a support, since there is another way to learn it (Abaye's way), and it is not a tenable proof, the way the Beraisa is (at least in the hava amina).
Thanks again, Tizku l'Mitzvos and a freilichen Zos Chanukah!
Yes, Daniel, you have understood my argument correctly.
1)
(a) That is indeed what I argued; that a Mishnah would not be brought as a support to Rabah, because Amora'im only state Chidushim not found in Mishnayos. However, afterwards I started thinking that neither Even ha'To'en (cited in my first answer) nor Ein Yehosaf (cited in my third answer) seem to agree with me on this, because if one looks into them carefully, they seem to understand that, in principle, all other things being equal, it would be possible to cite a support to Rabah from the Mishnah.
(b) Now I have found the Gemara in Kidushin 69b, which states that the Mishnah supports the opinion of Rebbi Elazar, who is an Amora. I also found a Tosfos in Shabbos 48a (DH, v'Chi) who asks why we the Gemara cites support from a Beraisa and not from the Mishnah. So we see that it is possible that the Gemara would cite a support from the Mishnah. However, I still do not retract entirely from my Peshat, because I think that it is very common that the Gemara tries to support an Amora from a Beraisa, but a lot less frequent to support him from a Mishnah, for the reason I suggested: the Amora'im are expected to know all the Mishnayos, but not expected to know all the Beraisos, since the authority of the Beraisa is less than the Mishnah.
2) In light of the above, the chief answer to your question, Daniel, is what you write, that this particular Mishnah was not brought as a support for Rabah since there is another way to learn it -- like Abaye.
Yasher Ko'ach Gadol,
Dovid Bloom