1) THE FIREBRAND THAT BURNS A FIELD
QUESTION: The Gemara concludes, according to Rava, that Rebbi Elazar and the Chachamim -- who argue about whether the owner of a dog that ignites someone else's property with a "Chararah" pays Chatzi Nezek or Nezek Shalem -- both follow the view of Sumchus who says that one pays Nezek Shalem in a case of Tzeroros. They argue only in a case in which the dog does an unusual act in the way that it ignites the property with the Chararah. Since there is a Shinuy, the act is a Toldah of Keren. The Chachamim rule that the owner pays Chatzi Nezek, as he does in any case of Keren. Rebbi Elazar rules that he pays Nezek Shalem because the dog damaged the property in the Chatzer of the Nizak, and Rebbi Elazar follows the ruling of Rebbi Tarfon that for damage of Keren in Reshus ha'Nizak one must pay Nezek Shalem mi'Gufo.
RASHI (DH Amar Lach) explains that when the Gemara says "d'Shani b'Tzeroros," that the dog did an unusual act, it means that the dog threw the firebrand with its paw onto the pile of straw (or other flammable material). The reason why Rashi explains that the dog threw the firebrand onto the pile is in order to make it a case of Tzeroros so that it should not be a case where the dog caused the damage directly, which would be a normal case of Keren. Rashi explains that it is a case of Tzeroros because the Gemara relates the case to the argument of Sumchus and the Rabanan, and says that the Tana'im of the Beraisa follow Sumchus. If it would not be a case of Tzeroros, they would not have to follow Sumchus, because one would pay Nezek Shalem for the damage done in the Chatzer ha'Nizak even without Sumchus' logic, because of the Halachah of Keren.
Why does the Gemara need to make this a case of Tzeroros altogether? If the discussion involves a dispute between Rebbi Tarfon and the Rabanan about whether Keren in Chatzer ha'Nizak pays Nezek Shalem or Chatzi Nezek, the Gemara should say simply that the case of the dog that damages a pile is where the dog simply placed the firebrand on the pile directly, with its mouth with a Shinuy (in an unusual manner), and the argument is whether normal Keren (which is not Tzeroros) pays Chatzi Nezek or Nezek Shalem when the damage is done in the Chatzer of the Nizak. Why does the Gemara need to say that the case involves Tzeroros at all? (TOSFOS 18b, DH Bein)
In fact, Rashi himself (DH d'Dachik Lah Alma) explains that this is a way of explaining the Machlokes between Rebbi Elazar and the Chachamim in the Beraisa. They argue in a case where the animal puts the firebrand directly on the pile and burns it, such that no Tzeroros is involved! The question is whether the owner pays Nezek Shalem or Chatzi Nezek for the area that the firebrand burned ("Makom ha'Gacheles").
Also, when the Gemara first suggests that the Machlokes between Rebbi Elazar and the Chachamim is the same as the Machlokes between Rebbi Tarfon and the Rabanan, and the case is where the dog did an unusual act, Rashi (18a, DH Ela Kegon d'Shani) explains that the Shinuy was that the dog took the firebrand in its mouth and placed it on the pile! The Rishonim explain that when the Gemara there says "d'Shani b'Gacheles," it is the exact same answer that the Gemara here -- at the end of the Sugya, according to Rava -- gives when it says "d'Shani b'Gacheles" (see Tosfos DH Bein, and see Chart, footnote #5). Why does Rashi there explain that the dog placed it on the pile instead of explaining that the dog threw it, as he explains here?
From the words of Rashi there it seems that the question does not involve Tzeroros, but rather an ordinary case of Keren, as he explains later (on 18b), and the Gemara is asking how much must the owner pay for the damage at the "Makom ha'Gacheles," the damage done to the place that the firebrand touched first.
Moreover, in the same comment there (18a, DH Ela Kegon), Rashi continues and says that the Machlokes between Rebbi Elazar and the Chachamim is about Tzeroros with a Shinuy! Rashi says that there is Tzeroros involved, even though the Gemara explains that the Machlokes is the same as the Machlokes between Rebbi Tarfon and the Rabanan. Why does Rashi say, in the beginning of his comments, that the dog placed the firebrand on the pile, and then, in the end of his comments, that the case refers to Tzeroros, seemingly a case where the dog throws the firebrand onto the pile (as Rashi explains on 18b)? (See SHITAH MEKUBETZES in the name of GILYON, PNEI YEHOSHUA, MAHARSHA in Mahadura Basra, MAHARAM SHIF.)
(a) TOSFOS answers that the Gemara (18b) explains that the Chachamim who argue with Rebbi Elazar follow Sumchus and that the question concerns Tzeroros of Keren and not just Keren, because the Chachamim in the Beraisa state simply that one pays Chatzi Nezek and they do not specify that one pays Chatzi Nezek only for the "Makom ha'Gacheles," the place that the firebrand touched and burned first. This implies that one pays Chatzi Nezek for the entire property that was burned.
However, the damage done to the rest of the field, besides the "Makom ha'Gacheles," is only damage of Tzeroros. Even if the dog placed the firebrand down onto field, when the fire spreads it should be considered Tzeroros, according to Rebbi Yochanan later (22a). Rebbi Yochanan and Reish Lakish there argue whether damage caused by one's flame is considered like "Chitzav," his "arrow" (Kocho), or whether it is considered his property (Mamono), like a person's animal. According to Reish Lakish, any damage caused by the firebrand is like damage caused by normal Keren, like an animal that damages, and the owner is liable to pay like in a case of normal Keren, and not like in a case of Tzeroros. According to Rebbi Yochanan, however, the Gemara there explains that when a person shoots an arrow, he is liable for the damage done, but when an animal "shoots" an arrow, the owner is liable because of Tzeroros. Therefore, if the dog places the firebrand on the pile and it burns the rest of the field, when the rest of the field burns it is through "Kocho," or Tzeroros, of the dog, and one would not be liable to pay Nezek Shalem according to the Rabanan who argue with Sumchus and maintain that for Tzeroros one pays Chatzi Nezek.
This is why the Gemara finds it necessary to explain that the case is one of Tzeroros of Keren, and not a case of ordinary Keren -- because of the implication of the Beraisa that there is Tzeroros involved, since the Beraisa seems to be discussing the rest of the field as well, which is certainly Tzeroros according to Rebbi Yochanan.
Perhaps Rashi agrees with this understanding, and thus he writes later that the Gemara chose not to explain that Rebbi Elazar and the Chachamim argue about ordinary Keren and not Tzeroros of Keren (and they are discussing the "Makom ha'Gacheles" alone), because the Gemara would then have limited the application of the Halachah of the Beraisa (and the Gemara prefers not to limit it). Rashi later indeed explains the Beraisa in such a manner only because he is forced to do so -- in order to show that Rebbi Elazar might not agree with Sumchus, and that Rebbi Elazar in the Beraisa is not discussing a case of Tzeroros at all.
Why, though, does Rashi (18a) mention that the dog took the firebrand in its mouth and placed it on the pile, and he does not mention that the dog threw it, even though he is discussing the same case (of "Kegon d'Shani b'Gacheles")? The GILYON cited by the Shitah Mekubetzes explains that Rashi there indeed is explaining that the Machlokes involves only the "Makom ha'Gacheles." Why, though, does Rashi there explain it in that manner? Perhaps he means to say what TOSFOS RABEINU PERETZ (in the Shitah Mekubetzes) says later -- that the Gemara at the end of the Sugya states clearly what this case (of "d'Shani b'Gacheles") refers to, and it says that the Machlokes between Rebbi Elazar and the Chachamim, according to Rava, is where the dog did an unusual act in the way that it did damage with the firebrand, and the Gemara states that both of the Tana'im in the Beraisa follow the view of Sumchus.
Why does the Gemara not state this earlier (18a), when it first mentions this case (of "d'Shani b'Gacheles")? (In fact, Rashi on 18a, DH Ela Kegon, does state this there as well.) Tosfos Rabeinu Peretz explains that there, the Gemara is satisfied with explaining that the Machlokes involves the "Makom ha'Gacheles" alone, and does not involve the rest of the pile and field. Therefore, the Gemara there does not need to say that it is a case of Tzeroros and explain that Rebbi Elazar and the Chachamim follow Sumchus. Only later, when the Gemara works with the conclusion of the Sugya and not merely with a tentative answer, does the Gemara find it necessary to give a more encompassing answer and to explain that Rebbi Elazar and the Chachamim argue not only with regard to the "Makom ha'Gacheles" but also with regard to the entire field. This is why the Gemara at that point says that both Rebbi Elazar and the Chachamim follow Sumchus, and they are discussing a case of Tzeroros of Keren.
This may be the intention of Rashi (on 18a) when he explains that the Gemara is discussing ordinary Keren, while here (18b) he explains that the Gemara is discussing Tzeroros of Keren.
However, this answer does not suffice to explain the words of Rashi. First, Rashi (18b) explains that what makes it Tzeroros of Keren is that the dog threw the firebrand onto the pile. Why does he not explain that the damage of the Tzeroros is, as explained above, the damage done to the rest of the field because of "Esho Mishum Chitzav"?
Second, as mentioned above in the question, Rashi (18a) contradicts himself. He first mentions that the dog placed the firebrand on the pile, but later he adds that the damage was done in the manner of Tzeroros of Keren, and that Rebbi Elazar and the Chachamim follow Sumchus, even though he is still explaining the answer of the Gemara at the beginning of the Sugya! Obviously, Rashi does not agree with the explanation of Tosfos Rabeinu Peretz.
(b) The MISHNAS KEHUNAH (cited in footnote 174 in the new printing of the Shitah Mekubetzes) asks how can the Gemara explain (on 18b) that the Tzeroros is the spreading of the fire to the rest of the pile and field, like Rebbi Yochanan's opinion? The Gemara there is discussing Rava, and TOSFOS in Sanhedrin (77a, DH Sof) writes that Rava rules like Reish Lakish who says "Isho Mishum Mamono" and not like Rebbi Yochanan. According to Rava, therefore, the burning of the rest of the field would not be a case of Tzeroros!
Perhaps Rashi was bothered by this question, and that is why Rashi here (18b) explains that the case is one of Tzeroros in that the dog threw the firebrand onto the pile. However, Rashi earlier (18a) explains that the dog simply placed the firebrand on the pile, and the Tzeroros is that the fire spread to the rest of the field (because "Esho Mishum Chitzav" is considered Tzeroros, like Rebbi Yochanan), because at that point the Gemara was not discussing Rava, and therefore there was nothing wrong with explaining the Gemara according to Rebbi Yochanan. (M. Kornfeld)
(c) Perhaps there is more to the words of Rashi earlier (18a). A careful reading of the words of Rashi shows that there are two separate comments combined into one. Rashi first explains the words of the Gemara, "Ela Kegon d'Shani b'Gacheles," and explains that the dog placed the Gacheles on the pile. Rashi then begins again with a new comment (with the words "Ela Ba'ayan me'Hacha Lo Tifshot...," with the word "Ela" being the "Dibur ha'Maschil") and he explains the Gemara with a different explanation. Perhaps there are two versions in the explanation of Rashi which correspond to the two versions in Rashi later (on 18b), as follows:
Rashi (on 18b, DH Amar Lach Rava) asks why the Gemara insists that if the Machlokes between Rebbi Elazar and the Chachamim revolves around the question of Keren in Chatzer ha'Nizak, then they both follow Sumchus. Perhaps they both follow the Rabanan, and still one pays Nezek Shalem for damage done by Keren in Chatzer ha'Nizak!
(Tosfos and other Rishonim write that according to the Rabanan who argue with Sumchus, one would not pay Nezek Shalem in the Chatzer of the Nizak, even according to Rebbi Tarfon, since Rebbi Tarfon learns his Halachah only from a Kal va'Chomer: if one pays Nezek Shalem for Shen and Regel in Chatzer ha'Nizak even though one is exempt for Shen and Regel in Reshus ha'Rabim, then certainly for Keren, for which one is liable in Reshus ha'Rabim, one must pay Nezek Shalem in Chatzer ha'Nizak. According to the Rabanan who argue with Sumchus, one would never pay Nezek Shalem for damage of Shen and Regel done through Tzeroros, similar to the damage of Keren (which the Gemara is discussing) which was done through Tzeroros, even in Chatzer ha'Nizak. Therefore, the Gemara must explain that Rebbi Elazar and the Chachamim follow Sumchus so that Rebbi Tarfon can rule that one is liable to pay Nezek Shalem for Keren in Chatzer ha'Nizak. Rashi here, however, is consistent with his own view and does not accept this, since later (DH d'Dachik Lah; see following Insight) Rashi writes either that one must pay Nezek Shalem for Tzeroros of Shen, or that there is no such thing as Tzeroros of Shen; in either case, one is always liable to pay Nezek Shalem for Shen in Chatzer ha'Nizak, and one will never pay only Chatzi Nezek. Therefore, the Kal va'Chomer is still valid according to the Rabanan who argue with Sumchus; they can make a Kal va'Chomer from Shen: if one pays Nezek Shalem in the Chatzer ha'Nizak even though one is exempt from Shen in Reshus ha'Rabim, then certainly one should have to pay Nezek Shalem for Keren, and Tzeroros of Keren, in Chatzer ha'Nizak. We cannot say that Tzeroros of Regel should refute the Kal va'Chomer, since one pays Chatzi Nezek for Tzeroros of Regel in the Chatzer ha'Nizak, since we cannot ask a Pircha from a Halachah that is known from a Halachah l'Moshe mi'Sinai.)
Rashi answers that it was necessary to explain that Rebbi Elazar and the Chachamim follow Sumchus because if they follow the Rabanan, then why do the Chachamim say that one must pay Chatzi Nezek for Tzeroros of Keren? In the Gemara later, Rav Ashi asks that perhaps one must pay only a quarter of the Nezek for Tzeroros of Keren. Since the Gemara here does not want to get involved with Rav Ashi's question, it therefore says that the Chachamim follow Sumchus, who certainly rules that one pays Chatzi Nezek for Tzeroros of Keren, as one pays for normal Keren.
This answer of Rashi, however, seems appropriate only according to the second Lashon of Rashi, in which he explains that the question of whether or not an animal that damages through Tzeroros can become Mu'ad (which was discussed earlier in the Gemara) is a question involving all cases of Tzeroros and not only Tzeroros of Keren. Accordingly, it is possible that Rava -- even if he rules that for Tzeroros of Keren one pays a quarter of the Nezek -- still could ask what the Halachah is in the case of normal Tzeroros of Regel: does the animal become Mu'ad, obligating the owner to pay Nezek Shalem, after it damages through Tzeroros three times? It is possible that for Tzeroros of Keren one pays a quarter of the Nezek the first three times and then pays Chatzi Nezek, while for Tzeroros of Regel one pays Chatzi Nezek the first three times and then pays Nezek Shalem. The Halachah l'Moshe mi'Sinai states simply that we delay the obligation of normal restitution until after the animal has damaged in this manner (Tzeroros) three times. That is why Rashi can say that it is possible that Rava, who was in doubt whether Tzeroros can become Mu'ad, may agree with Rav Ashi who says that perhaps one pays a quarter of the Nezek for Tzeroros of Keren.
(It is true that the Gemara later (beginning of 19a) says that if Rava rules "Yesh Shinuy l'Tzeroros l'Revi'a Nezek," it would be clear to him that Tzeroros could not become a Mu'ad. Nevertheless, Rashi (on 18b) explains that even if Rava rules "Yesh Shinuy... l'Revi'a Nezek" it is still possible that he still questions whether Tzeroros can become a Mu'ad in the case of Tzeroros of Regel. There is a logical way to explain that approach as well, in contrast to the Gemara's approach on 19a.)
However, according to the first Lashon of Rashi, the question of whether Tzeroros can become Mu'ad refers specifically to Tzeroros of Keren. If the Gemara wants to explain the Machlokes between Rebbi Elazar and the Chachamim in the Beraisa according to Rava -- who questions whether or not one pays Nezek Shalem for Tzeroros of Keren -- then obviously it cannot question whether for Tzeroros of Keren one pays Revi'a (a quarter) Nezek! The two questions are mutually exclusive: if one pays Revi'a Nezek for Tzeroros of Keren, then after three times he cannot pay more than Chatzi Nezek. On the other hand, if Rava is asking whether for Tzeroros of Keren one pays Nezek Shalem or Chatzi Nezek, then obviously he does not entertain the possibility that for Tzeroros of Keren one pays Revi'a Nezek. Why, then, would the Gemara have to say that Rebbi Elazar and the Chachamim follow Sumchus, and that the Chachamim maintain that for Tzeroros of Regel one pays Nezek Shalem and for Tzeroros of Keren one pays Chatzi Nezek? Even if the Chachamim follow the Rabanan who argue with Sumchus and say that for Tzeroros of Regel one pays Chatzi Nezek, for Tzeroros of Keren one would also pay Chatzi Nezek, according to Rava who questions whether one pays Nezek Shalem or Chatzi Nezek for Tzeroros of Keren after it damages three times! Rava clearly would say that after the first time, one does pay Chatzi Nezek for Tzeroros of Keren; one cannot assert that Rava wanted to avoid the question of Rav Ashi, because by definition his question already sides with Rav Ashi's question and clearly shows that he assumes that Tzeroros of Keren Tam pays Chatzi Nezek and not Revi'a Nezek.
Therefore, there must be another way to explain the Gemara. Why does the Gemara explain that Rebbi Elazar and the Chachamim both follow Sumchus? (TOSFOS HA'ROSH and TOSFOS RABEINU PERETZ cited by the Shitah Mekubetzes ask a similar question on Rashi's explanation.)
Perhaps the explanation of Rashi may be understood based on the words of TALMIDEI RABEINU PERETZ in the name of "Yesh Metartzim." They explain that the Gemara does not mean that Rebbi Elazar and the Chachamim rule specifically like Sumchus. Rather, it means that they even could rule like Sumchus. The Gemara means that it makes no difference whether they rule like Sumchus or the Rabanan, since they are not discussing Tzeroros of Keren. They are discussing a case of ordinary Keren, where the dog placed the firebrand on the pile, and they are discussing only the "Makom ha'Gacheles" according to Rebbi Yochanan, or the entire pile and field according to Reish Lakish. The Gemara does not mean that they are discussing Tzeroros of Keren.
If this is correct, it is clear that according to the first Lashon of Rashi the Gemara does not have to be discussing Tzeroros of Keren, but rather just Keren. That is why Rashi (on 18a) explains that the Machlokes between Rebbi Elazar and the Chachamim according to this answer -- which says that it depends on the Halachah of Keren in Chatzer ha'Nizak -- involves ordinary Keren (where the dog placed the firebrand there with its mouth) and not Tzeroros. When Rashi continues and says that the answer is discussing Tzeroros of Keren, he is following the second Lashon he writes later (on 18b). According to that Lashon, as explained above, the Gemara specifically seeks to establish the Machlokes according to Sumchus and not according to the Rabanan.
(The SHITAH MEKUBETZES cites the GILYON who even has this explanation as his Girsa in the words of Rashi on 18a, which reads that the Machlokes between Rebbi Elazar and the Chachamim is not according to Sumchus specifically, but that it is according to the Rabanan who argue with Sumchus, or, as explained above, according to the Rabanan of Sumchus and even according to Sumchus.)
This also explains what Rashi (18b, DH d'Dachik) means when he says that when Rava answers that Rebbi Elazar and the Chachamim both follow Sumchus, "he is just pushing off the answer." Rashi is saying that the Gemara does not mean that Rebbi Elazar must rule like Sumchus, but rather that Rebbi Elazar could even rule like Sumchus. (This answers the question of the RASHASH here.)
When Rashi (18b) says that the dog threw the firebrand, that is the same version of Rashi that explains that the Gemara is saying that Rebbi Elazar and the Chachamim only rule like Sumchus. That comment of Rashi is according to the second Lashon, and that is why he writes that the dog threw the firebrand and that it is a case of Shinuy (an unusual act) of Tzeroros (i.e. Tzeroros of Keren) and not just regular Keren. (M. Kornfeld)
(A general point in understanding how to relate to the first Lashon and the second Lashon of Rashi, and the many contradictions in the Sugya, is as follows. It seems that Rashi wrote a commentary on the Sugya and later modified his understanding and added his new way of learning the Sugya. The new way that he learned is the first Lashon, for that seems to be the commentary of Rashi after he modified is original understanding. Although Rashi writes in the second Lashon, "so I have heard and that is the Ikar," that does not mean that this is how Rashi himself prefers to learn. Rather, either he wrote that before he changed his mind, or he means that this is how most people learn the Sugya. This is evident from the fact that Rashi himself immediately asks a question on the second Lashon, showing that the first Lashon is Rashi's preferred one.
Such occasions of different, conflicting Leshonos in Rashi are found in other places (see Rashi 16a, DH Lo Kasha). In general, the rule is that the first Lashon is the one that Rashi prefers. He puts it first in order to indicate that we should learn the Gemara according to that explanation. The second Lashon is included out of respect for his teachers, or because that is how he wrote the commentary on the Gemara originally when he relied on the explanation of others, until he learned the Sugya again and decided to change his commentary. That is why, in the Sugya here, Rashi inserts the first Lashon before the second Lashon -- it is his conclusive explanation of the Sugya (that it is discussing Tzeroros of Keren when it asks whether Tzeroros can become Mu'ad). When Rashi added his new explanation, he also reviewed the Sugya and, in many places, in the beginning of a comment that discusses a point that is related to something that changed according to his new explanation, Rashi inserted a few words to explain the Gemara according to his new explanation, and this is, again, always at the beginning of his comment (for example, in the comment explained above on 18a, DH Ela Kegon). However, the old explanation still remains in the text of Rashi, since he did not take out the second Lashon. That is why, in many comments of Rashi, Rashi seems to contradict himself, switching back and forth between the first Lashon and the second Lashon, such that the MAHARSHAL finds it necessary to erase many words at certain points or to point out the variances (for example, see Rashi 18b, DH l'Keren, and DH O Dilma, and at the end of 19a; see also Insights there). In general, as mentioned above, the new explanation that Rashi adds is usually in the beginning of his comments.
However, Rashi did not re-write his comments in every single place that must be changed (slightly or considerably) based on his new, conclusive explanation. Therefore, in a number of places Rashi's commentary still clings to the second Lashon, meaning his original explanation, and he does not correct it. He just corrects his comments at the primary places which affect the Sugya. An example of this is on 6b, in DH Shor Re'ehu, where Rashi offers two explanations, and he explains the Sugya on 7a according to his second explanation, as pointed out in Insights there. Nevertheless, his conclusive explanation does seem to be his first explanation (that the Petur of Hekdesh is derived from Keren to all of the other Mazikin), since Rashi writes this explanation again in the Mishnah on 9b (DH Nechasim). That is why Rashi inserted it in the beginning of his comments, as the first explanation in the Sugya (and Rashi in Gitin writes only that explanation, as mentioned in Insights to 7a). Nevertheless, Rashi did not change his entire commentary on the Sugya on 7a, but he merely recorded the new, conclusive explanation in the place where it was most central and relevant to the Sugya.)
2) "TZEROROS" OF "SHEN"
OPINIONS: The Gemara discusses the Halachah of Tzeroros of Regel: the owner of an animal that does damage through Tzeroros of Regel pays Chatzi Nezek because of a Gezeiras ha'Kasuv. What, though, is the Halachah of Tzeroros of Shen? The Gemara discusses only Tzeroros of Regel and Tzeroros of Keren (in which the Tzeroros were kicked up in an unusual manner, as Rashi mentions at the beginning of 18b, and as the Gemara says at the end of 19a, according to the first Lashon of Rashi). However, Tzeroros of Shen is not discussed. Is there a Halachah of Tzeroros for Shen?
On one hand, the Gemara (17b) mentions as an example of Tzeroros a "Chazir she'Hayah Nover b'Ashpah" -- a swine that was shoving around with its snout in the garbage in order to find food, and it kicked out Tzeroros. The Gemara says that the owner, in such a case, must pay Chatzi Nezek. This seems to imply that Tzeroros of Shen is the same as Tzeroros of Regel.
On the other hand, when the Gemara earlier (3b) has a lengthy discussion in which it searches for cases of Toldos which are not similar to their Avos, the only case that it finds is that of a Toldah of Regel -- that case being Tzeroros of Regel, for Tzeroros is a Toldah which is not similar to the Av. The Gemara does not mention Tzeroros of Shen, which should also be a case of a Toldah which is not similar to its Av. This implies that there is no such Halachah of Tzeroros of Shen.
(a) RASHI (DH d'Dachik Lah Alma) points out that the Gemara here gives as an example of Tzeroros the case of "Hetilah Gelalim l'Isa" (the animal released its waste upon someone's dough), whereas the Gemara earlier (3a) taught that "Tinfah Peros l'Hana'asah" (the animal derived pleasure out of soiling someone's fruit) is a Toldah of Shen and pays Nezek Shalem. The case of "Tinfah Peros l'Hana'asah" seems to be a case in which the animal eliminated on the fruit and thereby ruined it; why, then, should that be different from the case of the Gemara here, which is a case of Tzeroros in which the owner pays only Chatzi Nezek?
In his first answer, Rashi explains that, indeed, if the animal would have derived pleasure from leaving its waste upon the dough, its owner would have had to pay Nezek Shalem, like a normal Toldah of Shen. However, in this case, the animal derived no pleasure from its act (the animal was ill and the waste came out by itself), and therefore the damage is not a Toldah of Shen. Rather, it is a Toldah of Regel and that is why the owner must pay only Chatzi Nezek.
It is clear from Rashi that any Toldah of Shen pays Nezek Shalem even if it damages via "Kocho," in a way similar to Tzeroros, rather than through direct contact between its body and the item that it breaks. It is only with regard to Regel that one must pay Chatzi Nezek for Tzeroros when the animal damages through "Kocho." Why, though, should that be so? The Gemara (3a) teaches that Shen and Regel are exactly the same with regard to obligating one to pay for damages! How, then, can their Halachos differ with regard to Tzeroros? The Halachah of Tzeroros should apply to both of them equally.
The answer is that the Halachah that Tzeroros pays Chatzi Nezek is a Halachah l'Moshe mi'Sinai. The Halachah l'Moshe mi'Sinai was given only with regard to Regel and not with regard to Shen. Therefore, for damage caused by Shen one pays Nezek Shalem since there is no Halachah l'Moshe mi'Sinai to teach that one pays any other amount. Moreover, the Halachah of Shen cannot be derived from Regel through a Binyan Av, because a Binyan Av, or Kal va'Chomer, cannot apply to a Halachah taught by a Halachah l'Moshe mi'Sinai (even if there would be no Pircha). Why, then, does the Gemara discuss whether there is Tzeroros of Keren? The Gemara discusses whether an animal that causes damage three times through Tzeroros of Keren becomes a Mu'ad or not; according to Rashi's first Lashon, the Gemara is asking whether the Halachah l'Moshe mi'Sinai -- that says that one pays Chatzi Nezek for damage of Tzeroros -- applies to Keren. From the Gemara later (19a) it seems that the question is whether the Halachah l'Moshe mi'Sinai was given only with regard to Regel or also with regard to Keren. If it was given with regard to Keren as well, then even after three times one will pay only Chatzi Nezek. (See Insights to 19a.) If, however, the Halachah l'Moshe mi'Sinai does not apply to Shen, why should the Gemara entertain the possibility that it applies to Keren?
The answer seems to be that, according to Rashi here, indeed the question of the Gemara is not only with regard to Keren, but with regard to Shen as well. The Gemara is asking whether there is a Halachah of Tzeroros for either Keren or Shen. The Gemara phrases this question with regard to Keren and asks whether one must pay Nezek Shalem after damage is done by Tzeroros of Keren three times. The same question, though, applies to Shen: does one pay Chatzi Nezek for Shen, as one pays for Regel, when it damages through Tzeroros, or not? Since Rashi at this point is explaining according to the Gemara's proposition that perhaps Tzeroros of Keren is like a case of a normal Keren and it can become Mu'ad after three times, he wants to explain that Tzeroros of Shen is also like a case of a normal Shen, just as Tzeroros of Keren is like a case of a normal Keren. However, according to the other view that Tzeroros of Keren cannot become Mu'ad and the Halachah of Tzeroros applies to Keren, it will apply to Shen as well, and the owner will pay Chatzi Nezek.
The Gemara earlier (3b) that says that the only known case of Tzeroros is Tzeroros of Regel also avoids the question of whether Tzeroros can become Mu'ad, meaning whether Tzeroros applies to things like Shen and Keren and not just to Regel, and that is why it mentions only the Toldah of Regel.
According to this explanation of Rashi, what does the Gemara (17b) mean when it says that a "Chazir she'Hayah Nover b'Ashpah" is considered Tzeroros? It should be a Toldah of Shen since the animal does damage by making Tzeroros through the process of eating, and therefore the owner should have to pay Nezek Shalem! The answer is that Rashi maintains that in that case, too, when the animal was shoving around in the garbage for food, it was not deriving pleasure from its act, and therefore the damage it caused is a Toldah of Regel.
(b) Rashi continues and cites another way to differentiate between the Sugya earlier (3a) which discusses "Tinfah Peros" and the Sugya here, which says that "Hetilah Gelalim" is Tzeroros, and he adds that this is the correct explanation. Rashi writes that "Hatalas Gelalim" is always pleasurable for the animal ("l'Hana'asah"). The reason why the Gemara here does not consider "Hatalas Gelalim" a Toldah of Shen is that one of the defining factors of Shen is that the animal has physical contact with the object that it damages. It cannot be considered Shen when the animal damaged the item merely by pushing something else that then pushed the object and damaged it (which is the case of "Hatalas Gelalim"). Therefore, any damage done through the animal's "Ko'ach" and not through direct contact with the item is always a Toldah of Regel and, by definition, cannot be a Toldah of Shen, because part of the definition of Shen is that the animal had pleasure from the object that it damaged through direct contact. This seems to be the way Rashi explains earlier (3a, DH Nis'chachah, and 17b, DH Tzeroros Ki Orchaihu). Why, then, does the Gemara question whether Tzeroros of Keren has the Halachah of Tzeroros or not? Keren should also be the same as Shen; it should only be considered Keren when the animal damages through direct contact and not through Tzeroros! Whenever it damages through Tzeroros, it should be considered a Toldah of Regel.
The answer might be that Shen is a form of damage done through the Hana'ah that the animal derives. That Hana'ah is not evident in the object that the animal pushes when it damages through Tzeroros. When the animal leaves its waste on an item, the animal's Hana'ah occurs only at the moment that the waste exits its body. Afterwards, when the waste damages someone's item, it is unrelated to the Hana'ah of the animal. Therefore, it is Shen only when the animal damages through direct contact with the object that it damages, such that the object that it damages is related to the Hana'ah that the animal derives at the time that it damages.
In contrast, the defining factor of Keren is that the animal acted in an unusual manner. Hence, even when the animal pushes an object in an unusual way, what the object then does is also related to something unusual. The object normally would not be flying through the air in this manner, since the animal normally does not kick it. Therefore, the object that is damaged through Tzeroros ("Kocho") of Keren might also be considered a Toldah of Keren, since the change in the normal manner is evident in the Tzeror (the rock that was kicked by the animal) itself. That is why the Gemara questions whether there is a Halachah of Tzeroros of Keren or that perhaps all cases of Tzeroros are a Toldah of Regel, since it is not damaging through direct contact.
According to this explanation of Rashi, it is also clear why the Gemara (3b) does not suggest that the case of a Toldah that is not like its Av is a case of Tzeroros of Shen, since there is no such thing as Tzeroros of Shen. The Gemara there does not mention Tzeroros of Keren because whether or not there is such a thing as Tzeroros of Keren is a question in the Gemara later.
Also, according to this explanation, it is also clear why a "Chazir she'Hayah Nover b'Ashpah" (17b) is considered a Toldah of Regel and not a Toldah of Shen. Damage caused through "Kocho" and not through direct contact cannot be considered a Toldah of Shen, as Rashi himself writes.
TOSFOS SHANTZ and TOSFOS RABEINU PERETZ (3a) also seem to understand that there is no Tzeroros of Shen when an animal leaves its waste on food, but for a different reason. They explain that in order for something to be a Toldah of Shen, the animal must receive Hana'ah from the damage that was done to the object. In the case of "Hatalas Gelalim," it makes absolutely no difference to the animal whether its waste falls upon the food or upon the ground; where it falls has nothing to do with the Hana'ah that the animal derives. The same is true in most other cases where an animal causes damage through an indirect act ("Kocho") which gives it Hana'ah; the Hana'ah it gets is not from the object which eventually becomes damaged from something that the animal pushed. Rather, the Hana'ah that it gets is from pushing the first item. Therefore, they explain, "Hatalas Gelalim" is not considered a Toldah of Shen.
In fact, these Rishonim also do not explain that the case of "Tinfah Peros" (3a), which is a Toldah of Shen, is where the animal rolled on food, as Rashi explains. Perhaps they explain it differently because it makes no difference to the animal whether it rolls on food or on the ground -- it simply likes to roll on its back. Therefore, that case also should not be considered a Toldah of Shen. Rather, they explain that the case of "Tinfah Peros l'Hana'asah" means that the animal sat down on the fruit either because the fruit was cold and the animal was able to cool itself off by sitting on the fruit, or because the fruit was soft and the animal wanted to rest on something soft. That is why the case of "Tinfah Peros" is a Toldah of Shen.
Hence, according to their explanation as well, it is clear why the Gemara (3b) does not mention Tzeroros of Shen as a case of a Toldah which is not similar to its Av. Similarly, the case (17b) of a "Chazir she'Hayah Nover b'Ashpah" is also a Toldah of Regel according to their explanation.
(c) TOSFOS (17b, DH Nover, and 18b, DH Bein), however, does mention the concept of Tzeroros of Shen. He clearly understands that Tzeroros can apply equally to any of the Mazikin (Keren, Shen, and Regel), and a "Chazir she'Hayah Nover b'Ashpah" is a Toldah (Tzeroros) of Shen.
How does Tosfos understand the case of "Tinfah Peros" (3a), in which the owner pays Nezek Shalem? Why is he not obligated to pay only Chatzi Nezek, like Tzeroros, when the animal damages through "Hatalas Gelalim"? Tosfos will have to explain that in the case there the animal damages in a way that is not Tzeroros, either because the animal rolled on the fruit (as Rashi explains), or that it sat on the fruit (as the Rash mi'Shantz explains), or, as the RI MI'GASH explains (cited by the Shitah Mekubetzes on 19a), that the animal was ill and its waste was a continuous flow that came directly from the animal to the fruit that it damaged, and therefore it is not similar to a pebble that shoots out from the animals foot since the waste is still touching the animal at the time that it damages the object.
Why does the Gemara (3b) mention only Tzeroros of Regel as a Toldah that is dissimilar to its Av? What about Tzeroros of Shen and Keren? The SHITAH MEKUBETZES and the TALMID HA'RASHBA V'HA'ROSH (2a) cite RABEINU YONAH who explains that the Gemara there mentions the example that is most common. Tzeroros of Regel is the most common case of Tzeroros, and that is why the Gemara mentions it. It is true, however, that there are also cases of Tzeroros of Shen and of Keren which are cases of a Toldah that is not similar to its Av. In fact, the Shitah Mekubetzes points out that this is why Rav Papa did not simply say that "there is one case of a Toldah that is not similar to its Av," but rather he said that "there are some Toldos that are not similar to their Avos"; he was alluding to the Toldos of Keren, Shen, and Regel which are not like their Avos, since Tzeroros applies to all three types of Nezikin.
According to Tosfos, why does the Gemara (18b and 19a) discuss whether the Halachah of Tzeroros applies to Keren? It should be obvious that it applies to Keren, just as it applies to Shen! The answer is that Tosfos explains those Gemaras differently, in accordance with his explanation here (DH b'Mu'ad) which is consistent with the second Lashon of Rashi (on 18b). He learns that the question of whether or not Tzeroros of Keren can become Mu'ad is not whether or not Tzeroros applies to Keren altogether, but rather it is a question concerning Tzeroros of Regel. Similarly, the Gemara (on 19a) is discussing Tzeroros of Regel when it questions whether one is exempt in Reshus ha'Rabim or not, and it is not discussing specifically Tzeroros of Keren.