On 89a Shmuel takes the position that you say Gud Asik even by mechitzos she'ainan nikaros-covered walls. Accordingly, in the shitas Chachomim one can carry throughout an individual roof by a gog bein hagagin, as long as he doesn't transfer an object from one roof to another. This is because through Gud Asik the walls of each individual property are viewed as extending upwards, thereby providing a seperation between each adjoining roof.
On 90a Shmuel holds, in shitas R. Meir that one is allowed to carry from roof to roof bu gog bein hagogin, that all the roofs combine to form one reshus that is greater than a beis saseim shelo hukaf l'deira and therefore the combined reshus takes on the status of a carmelis and one can only carry four amos on the roof.
The seemingly glaring difficulty with this second opinion of Shmuel is as follows: Why don't we say Gud Asik on the walls of each of the individual residences, as Shmuel said on 89a, thereby providing a seperation between each individual roof. This would then make each roof a seperate enclosed area of (assuming) less than a beis saseim which would allow free range of carrying all across the roofs (since each roof would now be considered a reshus hayachid and according to R. Meir one can carry from one roof to another)?
This question bothered me greatly because it seems from the two opinions of Shmuel that Gud Asik is a concept that is applied arbitrarily
, which with all the imaginary aspects of eruv that are already employed, to then add the element of arbitrariness is too much.
The Chazon Ish
answers that Shmuel really doesn't hold of Gud Asik by mechitzos she'ainan nikaros at all. That is why in fact the gogin, on 90a, join to make one large gog greater than a beis sasaim and turn the reshus into a carmelis, disregarding each individual residence's seperating walls beneath. On 89a Shmuel is explaining within the shitas Chachomim that we say "just like the residences are seperated below so too they are seperated above". Since the Chachomim view the inhabitants below as being transposed above, thereby prohibiting anyone from carrying from one gog to another, it only makes sense to view the seperating walls as being transposed upwards as well. In other words, Shmuel is saying on 89a that if the Chachomim view the individual residents below as living seperately above, then you also must view the walls below as providing a seperation between each gog above.
It isn't a Gud Asik issue, it is in fact a consistency issue, the very issue that I was bothered with in the first place.
(A side issue as well is do we say Gud Asik on interior walls of an individual residence. If Gud Asik by mechitzos mechusos is an absolute fact according to Shmuel, then why not say it even regarding the walls of each room in a residence. Yet by gog gadol next to gog katon we seem to disregard interior walls. I won't elaborate here. With the Chazon Ish's take this isn't a question.)