1) THE DOMAIN OF THE "NIZAK"
QUESTION: The Beraisa lists four categories of Halachah concerning Shen, Regel, and Keren: a Reshus which belongs to the Mazik (Reshus ha'Mazik), a Reshus which belongs to the Nizak (Reshus ha'Nizak), a Reshus which belongs to both the Mazik and the Nizak, and a Reshus which belongs to neither the Mazik nor the Nizak. The Gemara explains that the Reshus which belongs to both of them refers to a Chatzer into which they both have permission to bring their oxen or their fruits, such as a valley ("Bik'ah"). In such a place, one is exempt for damages of Shen v'Regel, and one is obligated to pay Chatzi Nezek for damages of Keren (Tamah). The reason one is exempt for Shen v'Regel in such a Chatzer is that it is considered like Reshus ha'Rabim, since the Mazik had permission to bring his Shor there. (See Rashi DH ud'Rav Yosef. The Rishonim explain that this is the logic behind the Halachah that one is exempt for Shen v'Regel in Reshus ha'Rabim. In a place where one's Shor is permitted to walk, he does not have to "hold on to its legs" and walk behind it to make sure that it does not trample the property of others (see Gemara 19b). Rather, people grant each other permission to walk their animals in a normal manner through public places, and they take responsibility to watch their items so that they should not be damaged by another person's Shor. See Rosh 1:1, and Shitah Mekubetzes 16a, "veha'Nachash," in the name of Gilyon.)
The Gemara asks what the Beraisa is referring to when it mentions a Reshus that belongs to neither of them, in which one is obligated for Shen v'Regel and one is obligated for Chatzi Nezek for Keren Tamah. The field must belong to the Nizak, since the Beraisa teaches that one is liable for damages of Shen v'Regel in such a field, and the law is that one is liable for Shen v'Regel only in the Chatzer of the Nizak.
Ravina explains that the Beraisa is referring to a Reshus that does not belong to both of them, but rather belongs to only one of them, the Nizak, with regard to placing Peros there. Both the Mazik and the Nizak, however, have permission to bring their oxen into the Chatzer. Since the Mazik does not have Reshus to bring his fruit there, it is considered Chatzer ha'Nizak with regard to Shen v'Regel, and thus the Mazik is liable. Since he does have permission to bring his Shor there, it is considered Reshus ha'Rabim with regard to Keren, and therefore he pays only Chatzi Nezek for Keren Tamah (even according to Rebbi Tarfon).
Why does Ravina consider such a Chatzer to be a Chatzer ha'Nizak with regard to Shen? As mentioned earlier, when a Shor does damage in a Bik'ah, what makes the owner exempt from Shen v'Regel is the fact that his Shor is permitted to be in the field. Wherever his Shor is permitted to walk, he does not have to "hold on to its legs" and walk behind it to make sure that it does not trample the property of others (see Gemara 19b). Therefore, in a Chatzer where both the Mazik and Nizak have permission to bring their oxen, the Mazik should be exempt from Shen v'Regel even though he cannot bring his fruit there! (TOSFOS DH Lo)
ANSWERS:
(a) RABEINU TAM emends the Girsa in the Gemara (and also the Gemara at the beginning of 16a) because of this question. He explains that the fourth case of the Beraisa is a Chatzer into which neither the Mazik nor the Nizak has permission to bring his Shor. (With regard to bringing their Peros there, Rabeinu Tam preserves the Girsa of our text, that only the Nizak has permission to bring his fruit there.)
Since the Mazik does not have permission to bring his Shor there, he is liable for Shen v'Regel, exactly as explained in the question above. Since the Nizak does not have permission to bring his Shor into that Chatzer, that Chatzer does not have the Chumra of Reshus ha'Nizak with regard to Keren, according to Rebbi Tarfon (when the Shor of the Mazik damages the Shor, and not the Peros, of the Nizak), and therefore one pays only Chatzi Nezek for damage done by his Shor Tam.
According to this Girsa, why does the Gemara explain that only the Nizak has permission to bring his Peros into the Chatzer? That is irrelevant; what matters is whether the Mazik or Nizak has permission to bring his Shor there! Even if both of them have permission to bring fruit there, the Mazik should be liable for Shen v'Regel since he cannot bring his ox there. Rabeinu Tam answers that Ravina wanted to explain the Beraisa even according to the opinion of Rebbi Zeira, who argues with what the above explanation and maintains that if the Mazik shares the Reshus with regard to putting Peros there, he is exempt from Shen v'Regel, even though he has no permission to bring his Shor there. Although the Gemara earlier refuted Rebbi Zeira's opinion, Ravina did not want the Beraisa to be a disproof to Rebbi Zeira.
The RA'AVAD in the Shitah Mekubetzes cites a third Girsa in which the Gemara explains that the fourth case of the Beraisa is a Chatzer which belongs to neither the Mazik nor the Nizak with regard to putting their Peros there and for putting their oxen there. Why, then, is it considered a Chatzer ha'Nizak to obligate the Mazik for Shen v'Regel? The answer is that both the Nizak and the Mazik have permission to use the Chatzer for other uses, such as to sit in the Chatzer. This solves not only the problem that Rabeinu Tam had with his explanation (the Gemara indeed does not limit the ownership, with regard to other uses, to the Nizak and not to the Mazik), but it also explains why the Beraisa calls this Chatzer "neither his (the Nizak's) nor his (the Mazik's)," instead of calling it "a jointly-owned Chatzer." According to Rabeinu Tam (and according to the Girsa of our text), the Chatzer does belong to one of them with regard to Peros, and thus the Beraisa should call it a "Chatzer that is not jointly-owned" rather than a Chatzer that belongs to neither. According to the Ra'avad, the Chatzer indeed belongs to neither of them with regard to both Peros and oxen.
(b) RASHI, however, records the Girsa as it appears in our text, that the Chatzer is jointly owned with regard to bringing oxen there, and, nevertheless, the Mazik is liable for Shen v'Regel. How is this to be reconciled with the Gemara earlier that teaches that the Mazik is exempt from Shen v'Regel if his Shor damages the Nizak's fruits in a Bik'ah since he is permitted to bring his Shor there?
1. The TOSFOS HA'ROSH, cited by the Shitah Mekubetzes, answers that according to Rashi two requirements are necessary in order to give a Chatzer the status of a Reshus ha'Rabim with regard to Shen v'Regel. The first requirement is that the Mazik must have permission to bring his Shor there. The second is that the Chatzer must belong to the Mazik with regard to Peros as well. If the field does not belong to the Mazik, either for bringing his Shor there or for placing his Peros there, it is still considered "Sdeh Acher" (the Nizak's field). A Bik'ah is considered Reshus ha'Rabim since the Mazik may bring both his oxen and Peros there. The fourth case of the Beraisa, however, is not considered Reshus ha'Rabim with regard to Shen, even though the Mazik is permitted to bring his Shor there, since the Mazik cannot bring his Peros there and therefore it is still called "Sdeh Acher."
(Rebbi Zeira, who argues with this point earlier in the Gemara, maintains that a "Sdeh Acher" is defined only by ownership with regard to Peros. Even if the Mazik does not have permission to bring his Shor there, as long as he has permission to bring his Peros there it is not considered "Sdeh Acher" and the Mazik is exempt from Shen v'Regel. According to this answer, and according to Rebbi Zeira, the reason why one is exempt for Shen v'Regel in Reshus ha'Nizak is simply due to a Gezeiras ha'Kasuv that teaches that the Torah "had mercy on him" and exempted him in some cases, as the Tosfos ha'Rosh writes in Kesuvos 41a.)
2. The NIMUKEI YOSEF, and the SHITAH MEKUBETZES in the name of TALMIDEI RABEINU YISRAEL, explain that since the Nizak has permission to bring fruits into the Chatzer while the Mazik does not, whenever the Nizak has fruits in the Chatzer the Mazik may not bring his Shor there. He has permission to bring his Shor there only when the Nizak has no fruit in the Chatzer. This is why the Beraisa teaches that the Mazik is liable for Shen v'Regel even though he has permission to bring his Shor into the field; at the time the fruit which the Shor damaged were in the field, the Shor was not supposed to be there.
According to this explanation, whether or not a field is a Reshus ha'Rabim with regard to Keren depends entirely on whether or not the Mazik had permission to bring his Shor into the field at the time the damage was done.
3. Ravina here is quoting Rava. Perhaps Rava agrees with Rebbi Zeira that the definition of a Reshus ha'Nizak with regard to Shen v'Regel depends entirely on who has permission to put Peros there. Ravina himself, who maintains that the definition of Reshus ha'Nizak with regard to Shen v'Regel depends on whether the Mazik has Reshus to bring his Shor there, would not agree with Rava's explanation of the Beraisa and thus would explain the Beraisa the way the Gemara originally suggests (that the Reisha expresses the view of Rebbi Tarfon, and the Seifa expresses the view of the Rabanan).

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