Gemara

Question: The Beraisa said 'this is Reshus ha'Rabim.' What does this come to exclude?

Answer: It excludes R. Yehudah's other case:

(Mishnah - R. Yehudah): (Chachamim enacted Pasei Bira'os. If four L-shaped corner-posts of a certain size surround a well, one may take from the well and carry inside the posts.) If Reshus ha'Rabim passes through the posts, it must be diverted.

Chachamim say, there is no need for this.

Eruvin 22b (Rav Dimi citing R. Yochanan): Eretz Yisrael is not a Reshus ha'Rabim to be Mechayev one who carries in it.

Suggestion: It is not Reshus ha'Rabim because the steep rocks of Tzur surround it on one side, and a furrow 10 Tefachim deep on another side.

Rejection: Bavel is between two rivers, Peras and Diglas (Euphrates and Chidekel (Tigris)). The entire world is between the oceans (yet we do not say that they are not Reshus ha'Rabim)!

(Ravin citing R. Yochanan): Steep inclines of Eretz Yisrael are not a Reshus ha'Rabim to obligate one who carries in them, for they are (very) unlike the encampment in the Midbar

(Mishnah): (Very steep) paths in Beis Gilgul (a place in Eretz Yisrael) or similar paths are Reshus ha'Yachid.

(R. Yanai): The paths must be so steep that a slave cannot carry a Se'ah (about 12 liters) of wheat and run in front of an officer.

Chachamim would not need to teach this. They say that Rabim are not Mevatel Mechitzos (Pasei Bira'os) even when it is convenient to pass through, so all the more so when it is inconvenient!

Rather, R. Yehudah taught this.

Rejection: (It is even like Chachamim.) Yehoshua loved Yisrael and established roads for them:

Whatever was convenient he gave to the Rabim. Whatever was inconvenient he gave to individuals.

24b: There was a Rechavah in Fum Nahara, one end opened to a path through vineyards. The path ended at the riverbank.

Question (Abaye): How can we permit it? (It was more than Beis Sa'atayim, and already Hukaf but not l'Dirah.)

We cannot make a wall on top of the bank (to join the path to the Rechavah) l'Shem Dirah, for a wall on top of a wall does not help.

Rishonim

Tosfos (22b DH Dilma): The Gemara discusses only inclines in Eretz Yisrael, due to Yehoshua's love of Yisrael. In Chutz la'Aretz, one is liable according to R. Yehudah. According to Rabanan, one is exempt even in Chutz la'Aretz, for the Rabim are not Mevatel Mechitzos. According to Rabanan, we should say that all of Eretz Yisrael is Reshus ha'Yachid through the ladders of Tzur and the descent of Geder! Also Bavel is surrounded by the Diglas! The Rabim are not Mevatel Mechitzos. R. Yochanan (Eruvin 67b) says that if one throws into an area surrounded by walls, even if it is 60 Beis Sa'im, he is liable. Surely there is no limit to the size! I answer that a Mechitzah not made through man is not so important. Rabanan agree that traffic is Mevatel it.

Poskim

Shulchan Aruch (OC 363:29): If one end of a Mavoy ends at the sea, and the other end ends at a wasteheap, it does not need anything [else to permit carrying inside].

Magen Avraham (30): This implies that the sea is considered a Mechitzah. This is difficult, for Tosfos asked that mountains (and rivers) should make Eretz Yisrael (and Bavel) Reshus ha'Yachid. He answered that natural Mechitzos are not so important, and Rabanan agree that traffic is Mevatel them. We must say that here we discuss a place where boats do not pass to the port. There, the sea is a Mechitzah. The same applies to other waters where ships do not pass. They are Mechitzos, like in Siman 356, and so says the Gemara (24b). Tosfos says that the river itself was a wall. Also the Aruch (Erech Ozel) says so. Based on Sof Siman 345, I can answer that this is only when the riverbank is 10 (Tefachim) deep. Rashi and the Rivash say so. If the riverbank slopes, and after four Amos of incline it is not 10 deep, it is not a Mechitzah (Sa'if 36).

Magen Avraham (40): Perhaps "Rabim" to be Mevatel Mechitzos must be 600,000. It seems that the Poskim hold like this, for they wrote Stam. (I.e. they did not mention that Rabim are Mevatel Mechitzos, for we do not have 600,000 in one place - Machatzis ha'Shekel)

Sha'arei Teshuvah (9): Chacham Tzvi (37) says that England, Askusiya and similar places surrounded by the sea, and they are much higher than sea level, one is liable for them due to Reshus ha'Rabim. One must fix roads and markets in them (to permit carrying in them), like in other cities that are not surrounded, for Rabim are Mevatel natural Mechitzos.

Nishmas Adam (on Chayei Adam, 2:71:9): Tosfos discusses only when all the Mechitzos are natural, e.g. the ladders of Tzur and the Diglas River. However, in one Mechitzah, even if it is natural, Rabim are not Mevatel it. The Gemara connotes like this. It answers for R. Yochanan that even R. Yehudah holds that when there are two proper Mechitzos, Rabim are not Mevatel the wall. If so, all the more according to Rabanan when there are three manmade Mechitzos (Rabim are not Mevatel the natural Mechitzah). We say similarly in 362:10. When all of the walls are through Tzuras ha'Pesach, it does not help. Here is no better.

Mishnah Berurah (118): One end of the Mavoy ends at the sea. The sea is considered a Mechitzah, for its bank is 10 deep. The water is not Mevatel the Mechitzah, like it says in Siman 356. The Magen Avraham says that here we discuss a place where boats do not pass to the port, for if they did, they would be Mevatel the Mechitzah. Several Acharonim disagree and say that boats are not Mevatel the Mechitzah of the sea bank. However, if one end of the Mavoy ends at a river and there is a bridge over it and many people traverse it, several Acharonim say that this is Mevatel the Mechitzah, and one must make Tzuras ha'Pesach at the place of the bridge. This is only when the bank of the river or sea is 10 deep. If it is inclined, and it is not 10 deep after four Amos, it is not a Mechitzah. It is like flat land.

She'alas Ya'avetz (Sof 1:7 DH umi'Kol): The Taz says that water is not a Mechitzah (because people walk on it when it freezes), even in summer. How does he understand the Gemara's question to exempt for carrying in Bavel, for it is between rivers? (They can freeze!) All agree that Rabim are not Mevatel the Mechitzos of manmade ditches. All the more so ice is not Mevatel them, even if the fourth Mechitzah is natural, or if it is a mountain.

Toras Chayim (22b DH Dilma): How can Tosfos say that a natural Mechitzah is not considered a Mechitzah so much, and Rabanan agree that Rabim are Mevatel it? We say that Rabanan surely hold that a mound that ascends 10 Tefachim within four Amos (along the diagonal) is a Mechitzah, for it is inconvenient. A mound is natural! The paths of Gilgal are very steep. Surely they are natural. It is difficult to distinguish and say that the ladders of Tzur and descent of Geder could not be manmade, so they are like animals, but people could make the paths of Gilgal.

Chasam Sofer (1:89): Do not say that since bridges are made for traffic (Rabim are Mevatel the Mechitzos). Tosfos (22b DH Dilma) toiled to explain why all of Eretz Yisrael is not a Reshus ha'Yachid. If we would say the above reasoning, Reshus ha'Rabim, i.e. markets or big roads, would be Mevatel the Mechitzos. The Magen Avraham said to see the end of the Siman (where he wrote that only 600,000 are Mevatel Mechitzos). If so, how do we answer for the Rambam and those who do not require 600,000? The Magen Avraham's distinction (between when boats do or do not pass) is wrong. Keneses Yechezkeil says that the Magen Avraham misunderstood Tosfos. Tosfos meant only that it is not considered a Mechitzah so much, i.e. for (permitting carrying in) a Karfef (an enclosure, if it was not for the sake of residence) more than Beis Sa'atayim (5000 square Amos). I disagree. Mid'Oraisa, the Shi'ur of Beis Sa'atayim has no significance. Rather, we cannot say that Eretz Yisrael, Bavel and all lands surrounded by mountains and rivers are Reshus ha'Yachid, for the entire world is surrounded by the Okainus (Atlantic) ocean, and even so the Torah forbade taking from a private Reshus ha'Yachid to the Midbar. We must say that one may not take from a Reshus ha'Yachid surrounded by walls to a Reshus ha'Rabim surrounded by natural Mechitzos, for Rabim are Mevatel natural Mechitzos such as the Okainus. We say so only when it is convenient to use, like in the Midbar. The cloud flattened the ground, and there were no mounds. Mounds are unlike the encampment in the Midbar, and Rabim are not Mevatel even natural Mechitzos. Rabim are not Mevatel manmade Mechitzos, whether or not they are convenient. This answers Toras Chayim's question against Tosfos.

Aruch ha'Shulchan (48): When rivers freeze in winter, surely they are Mevatel Mechitzos, for people walk there like on the land. The Taz forbids also in summer, because they freeze in winter. Many disagree. Some permit even in winter. According to letter of the law, it is forbidden in winter because the Mechitzah is Batel, and permitted in summer. Even if this is not Bitul Mechitzah, Rabim are Mevatel Mechitzos. Even though the Gemara says that Chachamim do not say so, Tosfos says that they agree that Rabim are Mevatel natural Mechitzos. Why is the sea a Mechitzah? The Magen Avraham says that this is where ships do not pass. This is difficult. Rather, ships are unlike traffic of the Rabim (Keneses Yechezkeil 2, and the Magen Avraham himself in 345:14). Some Gedolim say that Tosfos does not mean that Rabanan agree that Rabim are Mevatel natural Mechitzos. The Gemara regarding a mound clearly refutes this. Rather, Tosfos means that it is not a full Mechitzah regarding more than Beis Sa'atayim. It is not considered Hukaf l'Dirah (Keneses Yechezkeil 2). Some say that when it is inconvenient to use, Rabim are not Mevatel Mechitzos. Therefore, Rabim are not Mevatel a steep mound.