1) THE EFFECTS OF THE INNER CHATZER ON THE OUTER CHATZER
The Mishnah discusses the status of two Chatzeros, an inner one and an outer one (see Insights to 66:2
). In the Gemara, Rav Dimi explains that Rebbi Akiva in the Mishnah maintains that "Regel ha'Muteres bi'Mekomah Oseres" (the residents of the inner Chatzer prohibit the residents of the outer Chatzer from carrying -- even though they made an Eruv among themselves in the inner Chatzer and are permitted to carry in their own Chatzer), because the residents of the inner Chatzer tread through the outer Chatzer (Derisas ha'Regel). The only way the residents of the outer Chatzer may carry is by making one Eruv with the residents of the inner Chatzer.
Rav Dimi adds that the Chachamim maintain that even "Regel ha'Asurah bi'Mekomah Einah Oseres" -- even when the residents of the inner Chatzer did not make an Eruv and may not carry in their own Chatzer, the fact that they trample through the outer Chatzer does not prohibit the residents of the outer Chatzer from carrying.
The Gemara disproves Rav Dimi's assertion by demonstrating that the Chachamim in the Mishnah follow neither extreme. They do not maintain that both "Regel ha'Asurah" and "Regel ha'Muteres" prohibit the outer Chatzer (as Rebbi Akiva maintains), and they do not maintain that no type of Derisas ha'Regel (even "Regel ha'Asurah bi'Mekomah") prohibits the outer Chatzer (as Rav Dimi says the Chachamim maintain). Therefore, Ravin concludes that there must be three opinions: the two opinions mentioned by Rav Dimi (the two extremes), and the opinion of the Tana Kama of the Mishnah, who says that only "Regel ha'Asurah" prohibits the outer Chatzer.
What forces Ravin to conclude that the Chachamim are a third opinion who maintain that the inner Chatzer never prohibits the outer Chatzer, even when the inner Chatzer is "Regel ha'Asurah bi'Mekomah"? The Gemara just proved that there are two opinions in the Mishnah -- the opinion of Rebbi Akiva that the inner Chatzer's Derisas ha'Regel always prohibits the outer Chatzer, and the opinion of the Tana Kama that the residents of the inner Chatzer prohibit the outer Chatzer only when they are "Regel ha'Asurah." No opinion in the Mishnah maintains that the inner Chatzer never prohibits the outer Chatzer.
Why, then, does Ravin conclude that there are three opinions? He should simply say that Rav Dimi is incorrect and there are only two opinions (and the opinion of the Chachamim is that "Regel ha'Asurah," and not "Regel ha'Muteres," prohibits the outer Chatzer).
(a) The RASHBA and RITVA explain that when the Mishnah quotes the Chachamim (after it quotes Rebbi Akiva) who say, "Ein Derisas ha'Regel Osrasah" -- "[the inner Chatzer's] trampling does not prohibit [the outer Chatzer]," these four words imply that there is no such thing as Derisas ha'Regel prohibiting another Chatzer. It is from these words in the Mishnah that Ravin and Rav Dimi infer that one set of Chachamim maintains that no type of Derisas ha'Regel prohibits the outer Chatzer. This is a third opinion, distinct from the Tana Kama and Rebbi Akiva.
(b) The TOSFOS HA'ROSH explains that the Chachamim mentioned in the Mishnah after Rebbi Akiva must hold that Derisas ha'Regel does not prohibit the outer Chatzer for another reason. The Tana Kama of the Mishnah states that only "Regel ha'Asurah" prohibits the outer Chatzer, but not "Regel ha'Muteres." After the Mishnah quotes Rebbi Akiva's opinion, it quotes another statement of the Chachamim to the effect that Derisas ha'Regel does not prohibit the outer Chatzer. What does this statement add to the ruling of the Tana Kama? It must be that the Chachamim mean that even "Regel ha'Asurah" does not prohibit the outer Chatzer.