Pasei Bira'os – Eruvin 20a
(excerpted from The Contemporary Eruv)
R' Yehuda and Rabbanan (the other Sages) dispute the underlying basis of the device of “pasei bira’os” (“boards surrounding wells”). This dispute, in Eruvin 20a and 22a, concerns an enclosure consisting of four two-sided posts (of at least an amah width in each direction and ten tefachim height), which, taken together, form the corners of a square. The purpose of this enclosure was to allow travellers to Yerushalayim for the Yomim Tovim who had to camp on the road over Shabbos to access wells — that, because of their dimensions were reshuyos ha'yachid — on Shabbos. The pasei bira’os served to enclose an area around the well and render it a reshus ha'yachid. (There are several criteria that must be met in order to utilize pasei bira’os, see Nesivos Shabbos Chap. 14. Nevertheless, when and whether one may make use of them in practice is a separate question from whether they create a reshus ha'yachid d'oraysa in the area that they enclose. They do indeed, regardless of whether there is an actual well in the enclosed area or not.)
R' Yehuda held that pasei bira’os were not effective if they were positioned in the middle of a reshus ha'R'm, because asu R'm u’mevatlei mechitzta — masses walk through it, and masses negate walls
. Rabbanan, however, were of the opinion that the pasei bira’os are even effective when positioned in the middle of a reshus ha'R'm, because lo asu R'm u’mevatlei mechitzta — the masses walking through the walls do not negate them. The Rambam follows the opinion of Rabbanan. Other Rishonim disagree. For example, the Hashlama (printed in the Shabsai Frankel edition - Yerushalayim, 1975 - of the Yad HaChazaka) decides in favor of R' Yehuda's opinion. Hence, according to the Rambam, the requirement for doors on a reshus ha'R'm that has valid tzuros ha'pesach - or any other form of enclosure that suffices me’d'oraysa - is only d’rabbanan: Since, me’d'oraysa, the masses travelling through a wall do not invalidate it, a requirement to impede their movement must, perforce, be d’rabbanan - see Chazon Ish, ibid., 74:1-3.
The Mishkenos Ya’akov, however, in accordance with his view that the Halacha follows Yehuda’s opinion, denies that the Rambam’s intent is to rule according to Rabbanan. He asserts that the Rambam may have meant only to allow pasei bira’os on a highway that is not a true reshus ha'R'm (Orach Chaim siman 121, p. 119 in the Sheinberger -Yerushalayim 1960 - edition). He concedes, however, that none of the Rishonim understood the Rambam in this vein. Rather, they all understood that the Rambam accepted the view of Rabbanan.