Eruvin 90

Just as yesterday’s daf contained a debate between Rav and Shmuel about the application of גוד אסיק מחיצתא (i.e. where a partition or wall exists that is at least ten tefachim high, it is considered to virtually extend upwards), today’s daf (Eruvin 90b) contains a further debate between Rav and Shmuel about the application of a parallel halachic concept called פי תקרה יורד וסותם (i.e. that the beam of a roof virtually extends downwards to form a virtual partition between the roof and the ground).
In this case, Rav rules that where a pavilion is situated in an open field it is permissible to carry within the space of the pavilion. This is because he rules that in such a situation, we apply the halachic principle of פי תקרה יורד וסותם such that each of the four beams of the pavilion roof are considered to have a virtual wall between themselves to the ground thereby creating a closed space. However, Shmuel rules that in such a situation where the pavilion has no physical walls whatsoever, the principle of פי תקרה יורד וסותם does not apply. Consequently, according to Shmuel, carrying under the pavilion is forbidden beyond a distance of four amot.
Interestingly, beyond the laws of Shabbat and Eruvin, both principles of גוד אסיק מחיצתא and that of פי תקרה יורד וסותם are also invoked within the laws of Sukkah in terms of the possibility of virtually stretching walls further up, or virtually creating partitions from the beams of a Sukkah roof.
However, as Rav Soloveitchik points out (see Harerei Kedem Vol. 1 Ch. 106), the way in which these principles are applied with respect to the laws of Shabbat & Eruvin differ significantly from how they apply within the laws of Sukkah: As he explains, ‘the partitions discussed with respect to the laws of [carrying on] Shabbat differ from the walls that are required for [the law of dwelling in] a Sukkah. This is because the partitions for Shabbat are essentially required to encircle and to establish a boundary for a domain, while the walls that are required for a Sukkah are fundamentally required to operate as walls.’
Having pointed this out, Rav Soloveitchik then examines numerous examples from the Shabbat laws and the laws of Sukkah, and he explains that the distinction between partitions for the sake of boundary making, and walls for the sake of establishing a sukkah, is why these laws differ in terms of the different measures required in each case, and in terms of the weight of their halachic efficacy in each context.
Quite often, in the various conversations I have with different people or in the books and articles that I read, I encounter how halachic principles drawn from one halachic context are applied to an altogether different halachic context. On occasion such corollaries are intellectually coherent and halachically compelling, but on other occasions the halachic argument is weak – even if it sounds strong to the one making the argument.
Just as Shmuel ruled that we cannot always draw down a virtual partition from a roof beam, I would argue that we cannot always draw halachic conclusions by applying a halachic principle found in one area of Jewish law to another. In fact, I would go so far as to claim that a significant element of Gemara study is not only to learn about ‘how’ certain halachic principles apply, but also to consider ‘where’ such principles apply, and whether they apply in precisely the same way in all cases.
Ultimately, halachic disagreements – such as those between Rav and Shmuel about גוד אסיק מחיצתא and פי תקרה יורד וסותם – come to teach us that while a halachic principle may apply in some cases, this doesn’t necessary mean that it should be applied in all cases.