1) HALACHAH: ONE WHO WAS TAKEN OUT OF HIS "TECHUM" ON SHABBOS
OPINIONS: Rav and Shmuel (end of 42b) argue about whom the Halachah follows in a case in which a person was forcibly taken out of his Techum Shabbos to an enclosed area. Rav maintains that the Halachah follows Raban Gamliel who says that the person may walk throughout the entire enclosed area. Shmuel maintains that the Halachah does not follow Raban Gamliel in this case, and one may walk only within his four Amos. Shmuel agrees, though, that in the case of a person who is on a boat on Shabbos, the Halachah follows Raban Gamliel who says that he is permitted to walk throughout the entire boat (see following Insight).
What is the Halachah?
(a) The RI (cited by Tosfos DH Halachah) and the RAMBAM rule that one may walk throughout the area enclosed by the Mechitzos, as Rav maintains.
(b) The RASHBAM (cited by Tosfos) and RABEINU CHANANEL rule that one may walk only four Amos, except in a boat where one may walk throughout the entire boat, as Shmuel maintains.
2) HALACHAH: ONE WHO TRAVELS IN A BOAT ON SHABBOS
OPINIONS: Although Shmuel argues that the Halachah does not follow Raban Gamliel in a case in which a person was forcibly taken out of his Techum Shabbos to an enclosed area (see previous Insight), Shmuel agrees that in the case of a person who is on a boat on Shabbos, the Halachah follows Raban Gamliel, and one is permitted to walk throughout the entire boat.
Does this Gemara imply that one is permitted to travel on a boat on Shabbos?
(a) The RASHBAM (cited by Tosfos DH Halachah) says that one is permitted to travel on a boat on Shabbos, and one may even board the boat on Shabbos. Even though the Beraisa in Shabbos (19a) says that one may not embark on a boat within three days before Shabbos, the Rashbam explains that the Beraisa is expressing the view of Beis Shamai, while we follow Beis Hillel who permits boarding a boat before Shabbos, and even on Shabbos. (See MAHARSHA.)
(b) The RITZBA disagrees with the Rashbam. Even though the Beraisa in Shabbos may be expressing the opinion of Beis Shamai, Beis Hillel agrees that one may not board a boat on Shabbos. The Ritzba explains that boarding a boat is included in the decree against swimming on Shabbos. The Rabanan decreed that one may not swim in a body of water on Shabbos lest he inadvertently construct a flotation device, and the same decree applies to boarding a boat on Shabbos. Alternatively, the Rabanan prohibited boarding a boat on Shabbos because of the concern that one might steer the boat more than four Amos in a Karmelis. He may, however, board a boat before Shabbos. (The Ritzba limits this allowance to a boat that travels on a river. He prohibits boarding a boat on Shabbos that travels on the sea.)
(See Insights to Shabbos 19:2 for further discussion regarding the Halachah of traveling in a boat on Shabbos.)
3) THE HALACHOS TAUGHT BY ELIYAHU HA'NAVI
QUESTION: The Gemara relates that someone taught seven Halachos before Rav Chisda in Sura at the beginning of Shabbos, and he taught them again before Rabah in Pumbedisa at the end of Shabbos. The Gemara initially assumes that it was Eliyahu ha'Navi who taught the Halachos (because only Eliyahu is able to travel that far in one day). The Gemara concludes that it was "Yosef Sheida," Yosef the demon (who does not observe Shabbos).
The Gemara implies that had Eliyahu indeed taught the Halachos, his rulings would have been accepted. Similarly, we find in Berachos (3a) that Rebbi Yosi learned a number of Halachos from Eliyahu. The Gemara in numerous places leaves certain Halachic questions in doubt "until Eliyahu comes to resolve them for us" (see, for example, Sanhedrin 44a and Menachos 32a).
How are we to reconcile this with the teaching of RASHI in Shabbos (108a) who says that we may not rely on Eliyahu for Halachic questions involving matters of prohibitions (Isur v'Heter), but only for questions involving factual information? Moreover, the Gemara in Temurah (16a) teaches that we may not rely on a prophet even to remind us of a Halachah l'Moshe mi'Sinai that was forgotten. Why, then, may we rely on the Halachic teachings of Eliyahu ha'Navi?
(a) The MAHARATZ CHAYOS in Berachos (3a) explains that when Eliyahu ha'Navi (or any other prophet) teaches a Halachah as a prophecy, or Nevu'ah, from Hash-m, we do not accept that ruling. However, when he teaches the Halachah as his personal opinion of Da'as Torah, then we may accept it. When Rashi in Shabbos says that Eliyahu cannot teach us a Halachah, he means that Eliyahu cannot teach us Halachos as Eliyahu ha'Navi -- in his capacity as a prophet. However, he may teach us Halachos in his role as a Chacham. (See also BIRKEI YOSEF OC 32:4, TORAH TEMIMAH to Vayikra 27:216, and CHASAM SOFER, Teshuvos 6:98, as cited by the DEVAR YAKOV to Bava Metzia 114a.)
The Maharatz Chayos in Bava Metzia (114a) explains that when Eliyahu gives a reason and a source for his ruling, his ruling is no different from the ruling of any other of the Chachamim of the generation.
Similarly, the TOSFOS YOM TOV in Eduyos (8:7), when he explains why we may rely on Eliyahu's future rulings on Halachic questions, says that Eliyahu will give his reasoning and proofs for his rulings.
(b) Similarly, when Eliyahu merely relates a Halachah that was taught already by someone else, we certainly accept it. Only when he, as a prophet, teaches a Halachah that was never taught before do we not accept it. (See also Insights to Bava Metzia 114:1.)