1) A SUKAH ON TWO HORSES
QUESTION: In the Beraisa, Rebbi Meir and Rebbi Yehudah disagree about a Sukah built on top of an animal. Rebbi Meir says that the Sukah is valid, and Rebbi Yehudah says that it is invalid. RASHI explains that the Beraisa refers to a case in which one takes doors and places them flat on top of two horses in order to serve as the floor of the Sukah.
Why does Rashi say that the case involves two horses? The Beraisa says that the Sukah was built "on top of an animal," and not on top of two animals. (Indeed, TOSFOS says that the Sukah was built on top of one animal.) (ARUCH LA'NER)
(a) The ARUCH LA'NER answers that the Gemara later cites another case in which Rebbi Meir and Rebbi Yehudah disagree. In the case of a Sukah that has an animal as one of its walls, Rebbi Meir invalidates the Sukah while Rebbi Yehudah says the Sukah is valid. Abaye suggests that Rebbi Meir's reason is because he maintains that "we must be concerned for death"; that is, the Rabanan decreed that such a Sukah is invalid because of the concern that the animal might die, and one will be left without a Sukah with which to perform the Mitzvah (Rashi DH Gezeirah).
Rashi is bothered with Rebbi Meir's ruling in the case of a Sukah built atop an animal. If Rebbi Meir invalidates a Sukah whose wall is an animal because the animal might die, then why does he also not invalidate a Sukah that is built entirely atop an animal? He should invalidate the Sukah in that case as well, out of concern that the animal might die and the Sukah will collapse! Therefore, Rashi explains that in this case, there are two horses. There is no concern that two animals will die, as the Gemara in Yoma (13a) says, "for the death of one we are afraid, but for the death of two we are not afraid." If one of the two animals dies, the owner of the Sukah will still be able to set the Sukah atop the animal that remains.
This answer is difficult to understand, because in the case of a Sukah atop an animal, the Sukah is whole even without the animal below. If the animal dies, one simply can take the Sukah and place it on the ground, and thus there is no reason to invalidate the Sukah just because of a fear that the animal will die. (If, for some reason, the Sukah must rest on an animal, he can find another animal upon which to place the Sukah.) Furthermore, the Mishnah (22b) clearly states that a Sukah upon "a camel" is valid, and the Gemara says that the Mishnah there expresses the view of Rebbi Meir. Why, then, does Rashi say that the Beraisa refers to a Sukah built on two animals?
(b) Perhaps Rashi is bothered by a different problem. The Mishnah discusses a Sukah built upon a "camel." Why does it not use the same wording as the Beraisa and discuss a Sukah built upon an "animal" ("Behemah")? Why does it specifically mention a camel?
The answer might be as follows. The Gemara (10a) teaches that a Sukah is valid only when its floor is strong enough to support the people and furnishings inside it. An ordinary animal is not strong enough to support an entire four-walled Sukah, together with the Sechach, beds, table, and people inside it. However, a camel is strong enough. By mentioning a camel, the Mishnah teaches that the animal upon which the Sukah is built must be strong enough to support the Sukah.
Why, then, does the Beraisa mention only an "animal" and not a camel? The answer is that a Sukah on top of another type of animal is valid if one finds a way to support the Sukah and all of its contents on top of the animal's back. In what case is an ordinary animal strong enough to support a Sukah and its contents? Rashi explains that such a case exists when one uses two animals instead of one. (The word "Behemah," that the Beraisa uses, can also refer to "animals" in the plural form, as in Yonah (4:11), "u'Vehemah Rabah.") (M. Kornfeld)
2) A SUKAH THAT WALKS
QUESTION: Rebbi Yehudah maintains that a Sukah built on top of an animal is invalid. The Gemara explains that his ruling is based on the principle derived from a verse that requires that a Sukah be fit for use all seven days of the festival. A Sukah built on top of an animal is not fit for seven days.
The Gemara earlier teaches that Rebbi Yehudah requires that a Sukah be a "Diras Keva" (7b, 21b), a permanent dwelling place. A Sukah that moves is not a "Diras Keva," as Rashi writes (7b, DH b'Rosh; 21b, DH she'Ein Lah Keva; 22b, DH ha'Oseh). Why, then, does the Gemara need to give a different reason for why Rebbi Yehudah invalidates a Sukah built atop an animal? The Gemara should say that his reason is because such a Sukah is not a "Diras Keva." Why does Rebbi Yehudah need the principle that a Sukah must be fit for all seven days of the festival?
(a) The RAMBAN (in Milchamos, 21b) disagrees with Rashi and says that the fact that a Sukah moves does not make it a "Diras Arai." It is still considered a "Diras Keva." Wherever the animal goes, the entire Sukah goes with it, and thus it is considered "Kavu'a" (permanent) in its place atop the animal.
Perhaps Rashi agrees that a Sukah is considered a "Diras Keva" even though it moves about (as Rashi (7b) implies, it only looks like it is "Arai," but it is not actually "Arai"). Even though Rashi elsewhere (loc. cit.) states clearly that a moving Sukah is not considered "Keva," Rashi may understand that this is the subject of a Machlokes Amora'im. Two Amora'im (on 21b) argue with regard to Rebbi Yehudah's reason for invalidating a Sukah built atop the legs of a bed. According to the first Amora, the reason is because such a Sukah is not a "Diras Keva" because it moves. Perhaps the other Amora, who gives a different reason, maintains that a moving Sukah is considered a "Diras Keva." The Gemara here gives a different reason for Rebbi Yehudah's ruling (for why a Sukah atop an animal is invalid) in order to satisfy the second opinion there.
(b) Alternatively, perhaps the verse that the Gemara here cites as Rebbi Yehudah's reason for invalidating a Sukah built atop an animal is itself the source for Rebbi Yehudah's opinion that a Sukah must be a "Diras Keva." The fact that the Torah requires a Sukah to stand for seven days teaches that a Sukah must have a quality of permanence. (M. Kornfeld)
(c) TOSFOS (DH Al Gabei) suggests that the case of a Sukah atop an animal refers to Sechach that is supported by poles that reach the ground. The animal supports only the floor and the walls of the Sukah. If the animal walks away (with the floor and walls of the Sukah), the Sechach of the Sukah will remain in place, and thus the fact that the Sukah moves does not make it a "Diras Arai" (since the Sechach remains stationary). Therefore, the Gemara gives another reason for why the Sukah is invalid according to Rebbi Yehudah.