When two distinct domains in which people live both have access to an area that is separate from them, and they would each like to carry back-and-forth from it on Shabbat, how do we decide which domain it belongs to?
Obviously, if the two domains are situated so that they can join together and make an eiruv, this area will be included in it and the residents of both domains will be allowed free access to it. If such an eiruv was not made, the Mishna (83b) teaches a basic principle: it is the domain that can access the area more easily that will be considered connected to it. One example that the Mishna brings is a huliyat ha-bor – a pit surrounded by a wall.
The embankments that surround a cistern or a rock that are ten handbreadths high may be used by the balcony; if they are lower than that height, the right to use them belongs to the courtyard. The Gemara assumes that the phrase to the balcony is referring to the residents of an upper story, who access their apartments through the balcony. The mishna indicates that if one set of residents can make use of a place by lowering and another set of residents can use it by throwing, the use of the place is granted to those who lower their objects, in accordance with the opinion of Shmuel and contrary to the opinion of Rav.
In an examination of the case of the pit with high walls around it, our Gemara asks whether it can be considered readily accessible to the people on the mirpeset (balcony), after all, the water in the pit is much lower down!? Rav Yitzhak son of Rav Yehuda explains that for this to work, we must be talking about a case where the pit was filled up with water to the very top. To the objection that in the course of Shabbat the water will be drawn from the pit, lowering its level below ten amot (cubits), he argues that since it was accessible – and permitted – when it was full at the beginning of Shabbat, it retains its status even after the water level drops. To this the Gemara objects that we should look at it the other way around – since it will not be readily accessible when the water level drops, making it forbidden, even at the beginning of Shabbat it should be forbidden.
This argument forces the Gemara to accept another approach to the problem.
The Ritva explains that Rav Yitzhak son of’ Rav Yehuda’s reasoning is based on a rule taught earlier in our massekhet (17a), that once an issue of eiruv on Shabbat was permitted, it remains permitted for all of Shabbat. An example of this is a case where two courtyards are separated by a wall with an opening in it that allows them to join together to make an eiruv. Even if that opening becomes closed up on Shabbat, the eiruv remains in force. The case in our Gemara is different because we know at the very beginning that the situation will change in the course of the day. Therefore, the rule that is more appropriate to apply is the one that teaches us to follow the situation at the conclusion of Shabbat rather than at the beginning of Shabbat.