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1. There is a dispute about whether Rebbi Yehudah maintains that a person may determine a status retroactively (i.e. through conditions).
2. The Gemara concludes that Rebbi Yosi maintains that one may not determine a status retroactively.
3. The Gemara discusses whether Rebbi Shimon maintains that in some cases one may determine status retroactively.
4. Rav Yosef: With regard to determining a status retroactively, one either always applies the concept or never applies it.
5. Rava: Rebbi Shimon always rules that one may determine status retroactively.
A BIT MORE
1. This dispute is relevant in many situations. For example, one may stipulate where his Eruv will take effect based on the occurrence of a future event. Ayo understands that Rebbi Yehudah does not allow this, whereas our Mishnah's version of Rebbi Yehudah implies that he does, and this is Ula's conclusion.
2. While some Beraisos say otherwise, the Gemara concludes these should be read differently to state that Rebbi Yosi maintains that a status cannot be determined retroactively.
3. When there seems to be a contradiction in Rebbi Shimon's opinion, the Gemara suggests that when Rebbi Shimon says that one may not determine status retroactively, he is referring to the status relevant to a d'Oraisa law, but with regard to a d'Rabanan law, he maintains that one may determine status retroactively.
4. Rav Yosef explains that one must either maintain that this principle is always applied (for both a d'Oraisa law and a d'Rabanan law) or that it is never applied.
5. Rava answers the contradiction in Rebbi Shimon's opinion (posed by the Gemara earlier) by saying that Rebbi Shimon was being stringent for a different reason and not because he maintained that a status cannot be determined retroactively.
Next Daf Index to Revach for Maseches Eruvin