Nedarim 45 - When partners fight
If two people jointly owned a courtyard - a widespread situation - and then they had a falling out so that each prohibited himself, by a vow, to benefit from his fellow's property, they have a problem entering their own courtyard. Since the courtyard is not divided, each portion of it is jointly owned by both. Thus, when one treads on the ground, he is partially transgressing the property of his fellow, which is prohibited to him by a vow.
However, Rabbi Elazar ben Yakov says that each one enters his own property when he enters the courtyard. When people buy property in common, they divide the ownership of it for use by area and in time. Exactly which portion and at what time belongs to each is determined by the future event of him entering it. So, when he comes into the courtyard, we say that this is precisely what they had agreed to, and now his action clarifies their agreement. This idea of retroactive designation, that is, "I see what happens, and I say that in the past we had actually agreed to exactly this division," is called "breira" or "clarification."
Thus, the argument between the Sages and Rabbi Elazar ben Yakov is whether or not we can apply "clarification" in this case. The Sages do not permit it, but Rabbi Yakov uses it because there is no other choice. Should the courtyard be larger so that they can divide? Even he would not allow them to use it until they split.
Art: A Dutch Courtyard by Pieter De Hooch
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