Legal possession is entirely dependent on physical possession, and thus the defendant, who maintains physical possession, is also in legal possession of the object. However, the dubious nature of the transfer of possession, is enough to prove the claimant's case. This approach subdivides regarding the details of the dubiousness of the transfer, which depend on the chosen textual variant.
Textual Variants – According to Rashi, the correct variant is to read "לא אמרן אלא בדברים העשוין להשאיל ולהשכיר אבל דברים שאין עשוין להשאיל ולהשכיר נאמן". Rashi understood this to be an additional limitation imposed on Rav Yehuda's statement, joining the four previous limitations. Thus, Rav Huna bar Avin's statement is part of an explanation of Rav Yehuda's statement.
What Are Objects Which May Be Lent or Rented? According to Rashi, these are any object which the owner is willing to lend or rent, in contrast to objects which are exclusively used by their owner (such as objects easily damaged by their use).
When is the Claimant Believed? According to Rashi and R"Y MiGash, the claimant is believed when he can bring enough proof that the disputed object was removed from his possession suspiciously. Only when all six conditions apply ([1] witnesses state the defendant hid the disputed object beneath his clothes, [2] despite it not being an object which tends to be hidden, [3] nor him being a person who tends to hide such objects, [4] the claimant does not usually sell his possessions, [5] the claimant is stating he lent or rented the object to the defendant and not that it was stolen, [6] the object is one which may be lent or rented), then the defendant is suspected enough that the claimant is believed.
Claiming the Object Was Stolen – According to this approach, unless the claimant can bring proof of a burglary, or of the defendant being a known thief, then he cannot claim the object was stolen. This חזקה, that the defendant is not a thief unless proven otherwise, is stronger than possible מיגוs to the contrary.
Necessity of an Oath – Rashi only mentions an oath (presumably the שבועת היסת) in the cases where the defendant is believed, in which case, in order to maintain possession, he must swear he bought the object honestly. When the claimant is believed, no mention is made of any oath on his part. This implies that שבועת היסת is limited to cases where the defendant (who is physical possession) is believed, and is taking the oath just as a precaution.
Rava's Case – According to R"Y MiGash, in Rava's case, witnesses stated that the book and scissors were removed from the claimants possession by the defendants' father, who hid them beneath his clothes.
1 Presumably, the case fulfilled all of the other criteria mentioned as well.
Textual Variants – According to R. Hai Gaon and R. Chananel, the text states "ובכולהו לא אמרן אלא בדברים שאין עשויין להשאיל ולהשכיר אבל דברים העשויין להשאיל ולהשכיר אינו נאמן". R. Chananel understood this to be limiting the prior discussion of Rav Yehuda's statement to cases where the object are not used for lending or renting, and provides an alternate explanation of Rav Yehuda's statement, which applies it also to all cases where the object is used for lending or renting. Thus, Rav Huna bar Avin's statement is an explanation of Rav Yehuda's statement.
What Are Objects Which May Be Lent or Rented? R. Hai Gaon and R. Chananel do not define these objects.
When is the Claimant Believed? According to R. Chananel, the claimant is believed when he can bring enough proof that the disputed object was removed from his possession suspiciously. This may happen in two ways: if the object tends to lent or rented, then it is enough that the defendant hid the object beneath his clothes to throw enough suspicion on the defendant. If, however, the object does not tend to be lent or rented, then five conditions are required for the defendant to be believed ([1] witnesses state the defendant hid the disputed object beneath his clothes, [2] despite it not being an object which tends to be hidden, [3] nor him being a person who tends to hide such objects, [4] the claimant does not usually sell his possessions, [5] the claimant is stating he lent or rented the object to the defendant and not that it was stolen).
Claiming the Object Was Stolen – R. Chananel does deal explicitly with cases where the claimant states the defendant stole the object (instead of claiming the object was lent). However, R. Hai Gaon implies, and Tosafot clearly understood R. Chananel to state, that the claimant is never believed when claiming the object was stolen (without positive proof), whether or not the object may be lent or rented. This חזקה, that the defendant is not a thief unless proven otherwise, is stronger than possible מיגוs to the contrary.
R. Hai Gaon states that when the claimant is believed, he must take an oath, that this oath is a form of the Mishnaic oath taken by a claimant who's property was stolen (and not the lower grade שבועת היסת).
2Rava's Case – According to R. Chananel, in Rava's case, witnesses stated that the book and scissors were removed from the claimants possession by the defendants' father, who hid them beneath his clothes. However, none of the other conditions need to have applied.
Legal possession is dependent on the facts of the case, and need not follow the physical possession. If physical possession is unlikely to have been caused by a sale, then legal possession returns to the original owner. Thus, in such cases, despite the defendant being in physical possession of the object, the claimant retains his original legal possession, which results in a ruling in his favor.
Textual Variants – According to Rambam, the text presumably states "ובכולהו לא אמרן אלא בדברים שאין עשויין להשאיל ולהשכיר אבל דברים העשויין להשאיל ולהשכיר אינו נאמן". Presumably, Rambam understood this to be limiting Rav Yehuda's statement to cases where the object are not used for lending or renting. Thus, Rav Huna bar Avin's statement is independent of Rav Yehuda's.
What Are Objects Which May Be Lent or Rented? According to Rambam, these are objects which are exclusively rented or lent to others (such as large containers rented for events, or special wedding jewelry), and are not used by the owner himself, in contrast to objects which are occasionally rented or lent out, but also used by the owner.
When is the Claimant Believed? According to Rambam, the claimant is believed whenever the disputed object is unlikely to have been sold to the defendant. Thus, if the object is only lent or rented, or when sheep graze unsupervised, or when the defendant acted suspiciously when he took the object (he hid it beneath his clothes despite it not being an object which tends to be hidden nor him being a person who tends to hide such objects, and the claimant does not usually sell his possessions) then the claimant is believed when he states he lent or rented the object.
Claiming the Object Was Stolen – According to Rambam, unless the claimant can bring proof of a burglary, or of the defendant being a known thief, then he cannot claim the object was stolen. This is despite possible מיגוs to the contrary.
According to Rambam, a שבועת היסת is necessary in all cases. When the defendant is believed, then the defendant swears a שבועת היסת that he owns the disputed object, and when the claimant is believed the claimant swears a שבועת היסת that the object is his. According to this, the Rambam understands that a שבועת היסת is required for people in legal possession of a disputed object, and not those in physical possession.
3Rava's Case – According to Rambam, in Rava's case, witnesses only stated that the book and scissors belonged previously to the claimant. This matches the two parallel mentions of the story, where no other details are mentioned.
Legal possession is dependent on the object, and need not follow the physical possession. If physical possession of a certain object can happen legally without a sale, then legal possession returns to the original owner (thus resulting directly in a ruling favoring the claimant). If, however, it is unlikely that that object will be transferred legally without a sale, then legal possession remains with the physical possessor, which will require proof that the transfer of possession is dubious enough, in order to override the legal possession (also resulting in a ruling favoring the claimant).
Textual Variants – R. Tam follows R. Chananel's variant, which states "ובכולהו לא אמרן אלא בדברים שאין עשויין להשאיל ולהשכיר אבל דברים העשויין להשאיל ולהשכיר אינו נאמן". R. Tam understood this to be limiting Rav Yehuda's entire statement to cases where the object are not used for lending or renting.
4 Thus, Rav Huna bar Avin's statement is independent of Rav Yehuda's.
What Are Objects Which May Be Lent or Rented? According to R. Tam, these are any object which the owner is willing to lend or rent, in contrast to objects which are exclusively used by their owner (such as objects easily damaged by their use).
When is the Claimant Believed? According to R. Tam, the claimant is believed whenever the disputed object is one which may be lent or rented, and the defendant cannot prove he bought it. Additionally, even if the object is one which may not be lent or rented, but the defendant acted suspiciously when he took it (he hid it beneath his clothes despite it not being an object which tends to be hidden nor him being a person who tends to hide such objects, and the claimant does not usually sell his possessions) then the claimant is believed when he states he lent or rented the object.
Claiming the Object Was Stolen – According to R. Tam, as a general rule, the claimant cannot make unsubstantiated allegations that the defendant is a thief. However, due to the mechanism of מיגו, if the claimant is believed when he claims the defendant rented the object from him, then he is believed when he claims the defendant stole the object from him, but only if such a claim does not result in the removal of the object from the possession of the legal possessor (מיגו להוציא). Thus, the claimant can only claim theft when the object is one which may be lent or rented (where the claimant is considered the legal possessor) and not otherwise.
5 R. Tam makes no mention of any oath.
Rava's Case – According to this approach, in Rava's case, witnesses only stated that the book and scissors belonged previously to the claimant. This matches the two parallel mentions of the story, where no other details are mentioned.