1) THE DIFFERENCE BETWEEN "BEREIRAH" AND A "TENAI"
QUESTION: The Gemara proves that Rebbi Yehudah maintains "Ein Bereirah" from the words of Ayo in a Beraisa. Ayo says in the name of Rebbi Yehudah that one may make an Eruv Techumin with a stipulation only with regard to one side of the town or the other. He may stipulate that whichever side of the town the Chacham visits is the side on which the Eruv should take effect. He may not say that if two Chachamim come (one on each side of town), then he reserves the right to choose, on Shabbos, the side on which his Eruv will take effect. In such a case the Eruv is not valid because the validity of the Eruv depends on Bereirah, and Rebbi Yehudah maintains that Bereirah does not work.
The Gemara points out that in both cases the person utilizes Bereirah. Why, then, is the stipulation in the second case not valid while the stipulation in the first case is valid? The Gemara answers that in the first case, the Chacham has already arrived to one of the sides of the town (before Shabbos), and therefore the location of the Eruv will not be determined by a future event. Rather, it is determined by the present location of the Chacham (which will only be known when word of his arrival reaches this town).
Why does the Gemara not suggest a very simple difference between the two cases of Ayo? Perhaps even when one stipulates, "If the Chacham comes to the east, my Eruv is to the east," the Eruv is valid, because it involves no more than a normal case of a "Tenai" (a "yes" or "no" condition). It is comparable to a case where one says, "If you do this act, this Get will be valid, and if you do not do this act, the Get will not be valid." In contrast, when the person says, "If two Chachamim come, I will choose tomorrow where I want my Eruv to take effect retroactively," there is no condition. The issue in that case is one of Bereirah, and thus the Eruv is not valid. Why does the Gemara consider the first case to be a case of Bereirah and not a normal case of a Tenai?
(a) RASHI in Gitin (25b, DH ul'Chi Mayis) writes that a condition, a Tenai, works only because the ability to fulfill the condition is in the hands of one of the parties involved, and it was the person's intention that the condition be fulfilled at the time that he stipulated the Tenai. (That is, because he plans to fulfill it and he wants the transaction to be consummated, the event that is contingent upon the completion of the Tenai takes effect immediately, even before the Tenai is executed. If the Tenai ends up un-fulfilled, then the event is uprooted retroactively. It is not possible to cause an event to take effect retroactively.)
This is not how the condition works in the case of Eruv Techumin. The fulfillment of the condition (that the Chacham comes to one side) is not in the hands of the person who makes the statement. Rather, it depends on the Chacham, who has nothing to do with the Eruv. Since the Eruv depends on an act that is not in the person's hands to fulfill, it is considered a case of Bereirah.
(b) The RAMBAN in Gitin (25b) argues with Rashi. He explains that a case of a Tenai exists when there are only two possibilities: either the event will occur or it will not occur. In contrast, when an event can occur in one of several ways it is not a case of Tenai. Rather, it is a question of Bereirah. The case of the Eruv that is dependent on the Chacham's arrival can occur in one of two ways (he could arrive in the east or in the west). Therefore, it involves Bereirah.
One may still ask, however, why is the case of the Chacham a case of Bereirah? Perhaps the person's stipulation involves two completely separate conditions. The first condition is that if the Chacham comes to the east, then the Eruv will be to the east, and if he does not come to the east, then the Eruv will not be to the east. The second condition is that if the Chacham comes to the west, then the Eruv will be to the west, and if he does not come to the west, then the Eruv will not be to the west. (See REBBI AKIVA EIGER, Ma'arachah #4 to Eruvin 38a. TOSFOS (end of Yoma 56a) in fact asks this question on the explanation of the Ramban.)
Apparently, the Ramban maintains that the establishment of the two Eruvei Techumin cannot be viewed as two independent events. Rather, one event takes place (an Eruv is made), and there are two possibilities as to how it will take place (to the east or to the west). The reason for this is that one cannot make two Eruvim to be Koneh Shevisah in two places (since a person lives in only one place at a time). Therefore, when the person adds that if the Chacham comes to the other side then his Eruv will be to that side, it is viewed as an addendum to his first condition. (See also Insights to Eruvin 36:2.)