I read the answer here about large eruvs and how islands and places surrounded by water are the simpler case, and it made me want to understand the whole sugya from the ground up.
1. The gemara in Shabbos (6b) and Eruvin (6a) talks about reshus harabim. Rashi says a real reshus harabim needs 600000 people passing through every day, like the camp of Yisrael in the midbar. Where does that number actually come from, since the Ramban and other Rishonim ask on Rashi that they find no source for it in Chazal? And the Rambam in Hilchos Eruvin perek 1 defines reshus harabim without ever mentioning 600000, and the Shulchan Aruch (OC 345:7) brings the 600000 only as a yesh omrim. So whose shita do we hold like, and how much can a city eruv rely on it?
2. If an island is surrounded by water on every side, isn’t the water itself a wall? Meaning is the whole island automatically one enclosed reshus hayachid, or does it still need an eruv of lines and posts around it? Same question for any city completely surrounded by water or a wall, like the gemara says about Yerushalayim that if its gates were not locked at night it would be a reshus harabim.
3. The part I understand least is the line. A wire on poles is not a wall, you can walk right under it. How does a tzuras hapesach turn a huge open area into a private place? And it sounds like in a real Torah reshus harabim even that does not help and you need actual doors that close, so what exactly is the line accomplishing?
4. So practically, what is an eruv actually doing? Is it changing the area from a karmelis into a reshus hayachid, or is the main part the eruv food joining everyone’s houses into one domain? What is the bottom line halacha for carrying today in a place that has no 600000?
The basic sugya is simpler than all the details make it sound. From the Torah there are essentially two relevant domains: reshut hayachid and reshut harabim. The Torah prohibits carrying from one to the other and carrying four amot in a reshut harabim. Chazal then created additional restrictions, particularly karmelit and the rules of chatzer and mavoi, so that people would not become accustomed to carrying and eventually carry in an actual reshut harabim. (See Shulchan Aruch O.C. 345; Aruch HaShulchan 345:1.)
What constitutes an actual Torah reshut harabim is the famous dispute. Rashi and the Behag require a place resembling the encampment in the Midbar: among the requirements are a road approximately sixteen amot wide and the presence of 600,000 people. The Rambam does not mention 600,000, and the Ramban and other Rishonim specifically questioned where that requirement appears in Chazal. The Shulchan Aruch first gives the definition without 600,000 and then brings the 600,000 requirement as יש אומרים. So this is a real machloket Rishonim, and much of the controversy over large city eruvin comes from it. My father’s position was always that a tzurat hapetach should not be used to “erase” a reshut harabim. If it is truly a reshut harabim d’Oraita, you need the type of genuine enclosure discussed by Chazal, including the issue of doors; a string around the neighborhood is not a magic solution.
So what is the string doing? A tzurat hapetach is not supposed to be a physical fence. Halachically it represents the form of a doorway… two sideposts and something across the top. Once Chazal allowed these rabbinically restricted areas to be enclosed by halachic walls and doorways, the top member did not have to be a two-by-four piece of lumber. It can even be something extremely thin, provided the posts and the top line satisfy the rules of tzurat hapetach. That is why today’s utility poles and wire can sometimes do the job. The Shulchan Aruch expressly says that even a very thin upper piece works. This is different from the korah of the classic mavoi, although historically the concepts are closely related and both are part of Chazal’s system for permitting carrying in areas that they themselves restricted.
Water is also not as automatic as saying, “It’s an island, therefore the whole island is one reshut hayachid.” A sea, riverbank, cliff, wall, or similar boundary can certainly constitute part of a halachic mechitzah, and that is why islands and naturally enclosed places are often much easier cases. But the Gemara itself says that if surrounding water automatically eliminated reshut harabim, then ultimately the whole world is surrounded by the ocean and there would never be a reshut harabim. Clearly it is more complicated than that. You have to examine the actual boundaries, openings, slopes and other rules of mechitzot in that particular place.
Finally, there are really two different things that people call “the eruv.” First are the mechitzot or tzurot hapetach that make the physical area one in which carrying can be permitted. Then there is eruv chatzeirot/shituf mevo’ot … the food that joins the Jewish residents together so that the separate houses and common areas are treated as one shared domain. The Rambam explains this explicitly: even though the entire courtyard is already a Torah reshut hayachid, Chazal prohibited carrying from the private houses into the shared courtyard until the residents make an eruv.
So the bottom line is: an eruv does not permit carrying in a definite Torah reshut harabim simply because somebody strung a wire around it. The normal city eruv works because we are dealing with areas that, based on the halachic definitions being relied upon, are not treated as a definite reshut harabim d’Oraita. Once we are in the world of the rabbinic restrictions that Chazal created, they also gave us the mechanisms … mechitzot, tzurat hapetach and eruv chatzeirot …. by which those restrictions can be satisfied.
The details can become extremely complicated, and I would not try to learn all of Hilchot Eruvin in one Q&A. If you have a particular street, neighborhood, island, or existing eruv that you want to understand, ask about that specific case and we can work through it.