Rav Hoshaya asks about a situation where the division between two courtyards collapsed on Shabbat, in effect adding new residents – the residents of the hatzer (courtyard) next door – to each of the courtyards.
Rav Hisda said: Come and hear a resolution to the dilemma from the mishna: With regard to a large courtyard that was breached into a small courtyard, it is permitted for residents of the large courtyard to carry, but it is prohibited for the residents of the small one to do so. It is permitted to carry in the large courtyard because the breach is regarded like the entrance of the large courtyard. Apparently, even if the breach occurred on Shabbat, it is prohibited for the residents of the small courtyard to carry.
Rava and Abayye argue that this ruling would only apply if the dividing wall fell down before Shabbat began. If however, the wall was standing at the beginning of Shabbat, we apply the rule that we have learned before (Eiruvin 17a) that once an area was declared permitted with regard to the rules of eiruvin it remains so until after Shabbat.
Tosafot point out that this rule does not apply in every case of eiruv, and it is important to distinguish between the case discussed in our Gemara, where the wall collapsed and we need to rule with regard to the relationship between the residents of each of the two courtyards, and a case where the wall between the courtyard and the reshut ha-rabim – the public domain – collapses. In that case we would rule differently, and the residents would not be allowed to carry in the hatzer.
This distinction stems from the fact that in our Gemara, when the dividing wall collapsed, the courtyards retain their basic designation as a reshut ha-yahid – a private domain – and all that has happened is that there are more residents than there were when the eiruv was established at the beginning of Shabbat. In such a case, we can apply the rule that once an area was declared permitted with regard to the rules of eiruvin it remains so until after Shabbat. In the second case, however, the collapse of the walls changes the courtyard from a reshut ha-yahid. It is no longer a simple hatzer. In such a case the entire situation with regard to the eiruv needs to be reevaluated.