QUESTION: Rebbi Nasan and the Rabanan argue whether an animal or bird is considered to "carry itself" ("Nosei Es Atzmo") when it is carried by a person or another animal. Rebbi Nasan maintains that an animal is Nosei Es Atzmo. The Rabanan maintain that it is not Nosei Es Atzmo, because it stiffens itself when carried and weighs itself down. Rebbi Yochanan says that Ben Beseira agrees with Rebbi Nasan that an animal is Nosei Es Atzmo. Ben Beseira states that one may sell a horse, that is used as a bird-carrier, to a Nochri on Friday before Shabbos, because there is no fear that the horse will do Melachah when Shabbos enters while still owned by the Jew. Since the horse is a bird-carrier, even if it carries birds on Shabbos for the Jew, it has not done a Melachah, because the birds that it carries are Nosei Es Atzmo.

Rashi explains that birds that ride on the back of an animal while they are not tied do not weigh themselves down. Therefore, the horse is not considered to be carrying the birds.

Rashi implies that Ben Beseira's ruling applies only with regard to birds that ride on the back of a horse; such birds do not weigh themselves down. However, the Gemara earlier says that the Rabanan (who disagree with Rebbi Nasan and Ben Beseira) maintain that an animal is not Nosei Es Atzmo because it weighs itself down. Why, then, do the Rabanan argue here (and maintain that a horse is considered to be carrying the birds on its back), if the birds are not tied and do not weigh themselves down?

ANSWER: The MAHARSHA answers that the Gemara later explains that Rebbi Nasan agrees that when the animal is tied up, it is not Nosei Es Atzmo and one (or one's animal) is prohibited to carry it on Shabbos. Rashi understands that even Rebbi Nasan agrees that when an animal is bound, it weighs itself down. That is, the dispute between Rebbi Nasan and the Rabanan involves exactly when the animal is considered to weigh itself down. According to Rebbi Nasan, only when it is bound does it weigh itself down, while the Rabanan maintain that it always ways itself down. According to this understanding, the question on Rashi is answered. When Rashi says that birds that ride on the back of a horse on their own volition do not weigh themselves down, he refers only to the view of Rebbi Nasan. The Rabanan, however, indeed maintain that birds atop a horse are not Nosei Es Atzmo and do weigh themselves down. (See MAHARSHAL for another approach to the words of Rashi.)