1) REBBI YEHOSHUA'S "MIGU"
OPINIONS: The Mishnah states that Rebbi Yehoshua agrees that a Migu may be used to support a claim in court in a case where the present owner of a field says to the original owner's son, "This field belonged to your father, and I purchased it from him" (he is believed because he could have remained silent and never said that the field belonged to the other man's father).
The Gemara explains the difference between this case and the cases in the other Mishnayos (12b, 13a) in which Rebbi Yehoshua says that a Migu cannot be used to support a person's claim in court (12b and 13a). The Gemara says that the difference between the cases is that in the case in the Mishnah here, "there is no slaughtered ox in front of you," while in the cases in which Rebbi Yehoshua does not agree that Migu works, "there is a slaughtered ox in front of you." What is the meaning of the allegory of a slaughtered ox, and how does it reflect a logical distinction between the different cases?
(a) TOSFOS (DH Hasam; 15b, DH u'Modeh; 18a, DH Iy) and other Rishonim give the most straightforward approach. Tosfos points out that there are two cases in which Rebbi Yehoshua does not apply a Migu. One case (in the Mishnah on 12b) is where the husband finds his newly married wife to be a Be'ulah, and she claims that she was not a Be'ulah at the time of the Erusin but that she was raped after the Erusin. The woman is not believed even though she could have said that she was a Mukas Etz (which would have been a better claim than saying that she was raped, because with the claim of "Mukas Etz" she would have been permitted to marry a Kohen). The second case is that of "Nistarah" (in the Mishnah on 13a) in which a woman secluded herself with an unidentified man and, when questioned, she said that although she had relations with the man he was Kasher and was not the type of person who would make her Pesulah to Kohanim. Rebbi Yehoshua says that she is not believed and we assume that she is Pesulah to Kohanim even though she has a Migu; she could have made a better claim and said that she did not have relations at all with the man. Had she made that claim, she would have benefited by retaining her status of a Besulah and getting 200 Zuz for her Kesuvah.
In both of those cases, even if she had claimed the better claim (which would have benefited her in some way more than her present claim), she would not have prevailed in court! In the case where the woman claims, "I was raped after the Erusin," if she would have said that she was a Mukas Etz she would still have not been believed according to Rebbi Yehoshua, because the only advantage of that claim is that it is "Bari" and her husband's claim is "Shema" (and, also, that she has a Chezkas ha'Guf to say that it happened after the Erusin). Rebbi Yehoshua, however, maintains that "Bari v'Shema" is not a reason to prevail in court (12b; Tosfos 13a, DH Rav Asi)!
Similarly, in the case of Nistarah, had the woman said that she did not have relations with the man, she would not have been believed, because Rebbi Yehoshua maintains, "Ein Apotropos l'Arayos" -- it is assumed that she had relations because no one watches to make sure that Z'nus does not occur (13b; RAMBAN here).
Rebbi Yehoshua rules that in these two cases, since the Migu is not a true Migu (since the woman would not have actually been believed with the claim of the Migu), we must suspect that the woman might be intentionally lying in order to win her case. She knows that had she said the better claim she would have lost the case, and therefore she says the weaker claim in order to win the case. This is unlike any Migu anywhere else in the Gemara (Ramban). In the case of every other Migu, there is nothing that the person gains with the weaker claim which he would not have gained with the stronger claim. It is only this type of Migu that Rebbi Yehoshua does not accept. This type of Migu is referred to as, "Shor Shachut Lefanecha," a "slaughtered ox in front of you," meaning that there was an act that was done which cannot be denied (and thus no claim will help the woman prevail in court).
In the case where the woman claims, "I was raped after the Erusin," the fact that she is a Be'ulah cannot be denied (it is like a slaughtered ox that cannot be revived). She has no way of winning her case by claiming that she is a Besulah; no claim she makes could win the case for her. Since it is known that she is a Be'ulah, there is no other claim that could win the case for her, and therefore it is not a real Migu.
Similarly, in the case of Nistarah, no claim that she could make could undo the fact that she secluded herself. Her seclusion is an undeniable fact. Therefore, she has no real Migu to deny that fact and to help her be believed in court.
In contrast, in the case of the Mishnah here in which the present owner of a field says to the son of the original owner, "This field belonged to your father, and I purchased it from him," the present owner could deny that the field ever belonged to the other person's father. By denying it, he certainly would win the case and be permitted to keep the field. Therefore, he has a full-fledged Migu when he says that he bought it. This is what is meant by "Ein Shor Shachut Lefanecha" -- "there is no slaughtered ox in front of you"; that is, there is no obvious fact here that cannot be denied.
(b) RASHI takes another approach to the Sugya (16a, DH she'ha'Peh she'Asar and DH Hacha; 17b, DH v'Lisni;, 18a, DH v'Nisni and DH Aliba d'Man). He explains that Rebbi Yehoshua accepts only a certain type of Migu -- a "Migu d'Iy Ba'i Shasik." Such a Migu exists when there is no claim being made against the person, but it is the person himself who initiates the claim against himself, while at the same time he dismisses it. In the case of the Mishnah here, nobody challenged the owner of the field, saying that it belongs to someone else. Rather, the owner himself informed the son of the previous owner that the field once belonged to his father and that he bought it from his father.
This is what the Gemara means when it says that in this case "there is no slaughtered ox" -- there is no claimant, there is nothing in front of Beis Din to provoke a response from the person who has the Migu. (Tosfos learns that the "slaughtered ox" represents a claim with a proof, and "no slaughtered ox" refers to a claim without a proof. In contrast, Rashi learns that the "slaughtered ox" represents a claim without a proof, and "no slaughtered ox" refers to when there is no claim at all.)
According to Rashi, it appears that whenever a claim is presented in court, Rebbi Yehoshua maintains that a Migu can never cause a claim to win.
There are difficulties with Rashi's explanation.
1. Tosfos disproves Rashi's explanation from the Mishnah which says that "if there are witnesses that the field once belonged to the father," the present owner cannot claim, without proof, that he bought it from the father. If, as Rashi says, Rebbi Yehoshua maintains that a claim alone suffices to refute the Migu, then why does the Mishnah need to say that if the son of the original owner of the field brings witnesses he is believed? He would be believed even without witnesses, as long as he comes with an independant claim!
Rashi might understand that this part of the Mishnah is not expressing solely the view of Rebbi Yehoshua. Rather, it is expressing a Halachah with which everyone agrees, even Raban Gamliel. Raban Gamliel, however, agrees that the original owner of the land is believed only when he has witnesses and not when he has merely a claim that the land belongs to him. (See also SHITAH MEKUBETZES, 15b, DH v'Li Divrei Rashi.)
2. The PNEI YEHOSHUA records another proof for the explanation of Tosfos from the Gemara earlier. The Gemara (13a) says that according to Rav Asi, Rebbi Yehoshua maintains that the woman is believed in a case of Nistarah, when she secluded herself with a man and she claims that he was Kasher, because she has a Migu that she could have said that she did not have relations at all. (According to Rav Asi, Rebbi Yehoshua does not maintain "Ein Apotropos l'Arayos," and that is why she would be believed had she claimed that she did not have relations.) According to Rav Asi, at least, it is clear that Rebbi Yehoshua does maintain that a Migu works to support one's claim even though there is a claim against her (there is a "slaughtered ox" in that case -- there is something present that provokes her response, and that is the fact that she secluded herself with a man). It must be, as Tosfos says, that Rebbi Yehoshua does accept a Migu whenever it is a true Migu and is not merely a better claim.
To uphold Rashi's explanation, the Pnei Yehoshua answers that perhaps Rashi would concede that according to Rav Asi, Rebbi Yehoshua accepts a Migu.
Another answer for Rashi may be that even Rashi agrees that Rebbi Yehoshua accepts a Migu for a woman who has a "Chezkas Heter" to permit her to marry a Kohen. Since she is not taking anything away from anyone and she is not making a claim that counters the Chazakah, she is believed. However, to take away money from another person, which involves overriding a Chazakah (i.e. that the other person, who is presently in possession of the money, is the owner), a Migu will not work.
In summary, according to Rashi it seems that Rebbi Yehoshua does not accept any of the Migus commonly found throughout the Gemara. (The RAMBAN rejects such an approach. He suggests an answer (which he admits is a very forced answer) to explain that even according to Rashi, Rebbi Yehoshua accepts an ordinary Migu, and only in this case does he not accept the Migu.)
Why does Rashi not accept the straightforward explanation of Tosfos? Rashi gives this explanation because of his reading of the Sugya later (18a; see Rashi there). Rashi infers from the Gemara later that Rebbi Yehoshua does not maintain that any normal Migu works. However, the question remains: how does the Gemara there know that this is Rebbi Yehoshua's opinion?
Apparently, the Gemara understands that this is the simplest understanding of the wording of Rebbi Yehoshua. In the Mishnah, Rebbi Yehoshua presents the case as, "One who says this field belonged to your father...," where the present owner initiates the question (and gives the answer) about the possession of the field. The Mishnah clearly implies that the Migu is accepted only when the person who has the Migu is the one who informs us that the field once belonged to the other's father. The simple reading of the Mishnah seems to support Rashi's explanation.
(c) The BA'AL HA'ME'OR has the opposite Girsa in the Gemara. In the case of the Mishnah here, there is a "slaughtered ox" in front of him and that is why he is believed. The Ba'al ha'Me'or explains that the "slaughtered ox" simply means that the "Chezkas Mamon" that one person used to have on the property has come to an end, and now the property is in the hands of someone else -- the person who says that he bought it. The Halachah states that in the case of a slaughtered ox, once a proper Shechitah has been performed it no longer has a "Chezkas Isur" (a Chazakah that it was forbidden either because of "Ever Min ha'Chai" or because it was lacking Shechitah). In contrast, the case where the woman claims, "I was raped after the Erusin," is compared to an ox that is not slaughtered; just as the ox is still in its original state of a "Chezkas Isur" (until we prove that it was slaughtered properly), the husband, too, still has a Chazakah on the money of the Kesuvah.