One is not allowed to travel two-thousand amot outside their city – referred to as the techum – on Shabbat or Yom Tov. If one wishes to travel beyond that distance in one direction they can move their “dwelling”, thereby relocating the centre of the permissible area that they can travel. We have learnt that one way to do this is by placing food, an eiruv techum, in the location of their new dwelling.
The Mishnah (3:5) taught that if one is unsure which direction, east or west, they need to gain that distance, for example to hear the shiur of a Chacham, there is a solution. They can place two eiruvei techum, and stipulate that if Chacham comes from one of the two directions, then the eiruv techum will be in that direction.
The Tosfot Yom Tov explains that this solution works based on the principle of bereira – literally, retroactive selection. In other words, the matter now can be clarified by a future event and be considered as if it is clear now. He continues that ordinarily we do not rule that bereira is valid or can be relied upon. However, since eiruv techumin is a rabbinic law, the Rabbis considered bereira valid for this law.
R’ Akiva Eiger (on our Mishnah)*raises an important issue. While we do not hold that bereira is valid for biblical laws, performing acts on conditions (tenai) are valid. For example, one could give his wife a get(divorce) conditional on here performing some task. The fundamental question therefore is what is the difference between a case requiring bereira and a regular tenai*?
R’ Akiva Eiger cites the Ramban who explains a regular tenai is when one stipulates about one thing. For example, if a person place only on eiruv to the east of the town and stipulate that it is only valid if his Rav comes in that direction, then it would be a valid tenai. However if one stipulates between two thing, like the two eiruvin in our case, then it would require bereira. To provide another example, if one told a scribe to write a get now for whichever of his two wives (that have the same name) who exits first, such a case would require bereira to be valid (Gittin 25b). (Since however this case would impact a biblical law, the use of bereira would not be allowed.)
R’ Akiva Eiger however is not satisfied with this explanation. In our case, the two eiruvin are not dependant on one another. The placement of each of the eiruvin should be considered independent from one another since each condition could be made separately without reference to the other eiruv techum. That being the case, it should be considered two separate and valid instances of a tenai. There should be no need for bereira!
The Garnat explains the Ramban in more detail that can be used to answer the question of R’ Akiva Eiger1. When someone stipulates about one thing, for example one performs kidushin (halachic engagement) with a tenai, that act that was performed was performed with certainty. The tenai that was made at the time of the act however stands in the way of the act having halachic force. If the tenai was annulled, then that kidushin would apply automatically.
If however one stipulates about two things, because the two acts are contradictory the acts themselves are in doubt. Consequently bereira is required to determine which of the acts are valid. Therefore in our case, since a person can only place one eiruv techum, placing one in the east and another in the west are contradictory; the very acts are questionable. Therefore bereira must be employed if we want to determine which eiruv techum was valid at the onset of Shabbat.
1 The explanation was found in Yalkut Bi’urim, Gittin 25b, Metivta, Oz Ve’Hadar. Also note that there are other explanations found in the Rishonim for the difference between a regular tenai and bereira.See Rashi (Eiruvin 25b) for example.
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