While discussing Rabbi Yehuda’s opinion about which we learned in yesterday’s daf, the Gemara introduces the concept of beraira – literally, a choice. This concept is raised with regard to a number of issues in Jewish law, and the basic question that it raised is whether a choice can be made after the fact to take effect retroactively. For example, if someone purchases wine just before Shabbat that had not been tithed, can he announce that what will be left over will be the tithe so that it can be drunk on Shabbat? In this case, tithing cannot be done on Shabbat, so it cannot be set aside now. Can we rely on what will be left over at the end to permit the wine to be drunk beforehand?
Another example of beraira brought by the Gemara involves the case of creating an eruv tehumin for the purpose of extending the area outside of a city where a person wants to walk on Shabbat. Generally speaking, on Shabbat, a person is not allowed to walk more than 2,000 cubits outside of the inhabited area where he lives. This Rabbinic injunction – which is based on Biblical foundations – can be altered for a variety of reasons if the person creates an eruv tehumin. This involves placing food for a meal at the edge of the 2,000 ama (cubit) limit, indicating that for this person Shabbat is being established in this place. By doing so, he will now be permitted to walk 2,000 amot from the eruv, beyond his original limit (it should be noted that he did not gain any freedom of movement, he merely transferred it from one side of the city to the other).
While the rule of eruv tehumin is accepted by all, there is a difference of opinion about a case where the resident wants to keep his options open. Can he place two eruvin, one on either side of the city, and say “if the guest scholar comes to the town to the east, I would like the eruv to the east to be effective; if he comes to the town to the west, then I would like the eruv to the west to take effect.” This scenario is presented by the Gemara as another example of beraira, where we will only find out what we want after the fact.
The halakha in these cases is that in cases of Biblical law we say that beraira does not work, but in cases of Rabbinic law we allow it to be applied.