1) ACHIEVING PERFECT PRECISION IN HUMAN ACTIONS
OPINIONS: Rebbi Yosi maintains that it is possible for natural occurrences to achieve perfect precision ("Efshar l'Tzamtzem bi'Yedei Shamayim"), and certainly it is possible to achieve perfect precision in human actions ("bi'Yedei Adam"). Why is it easier to achieve perfect precision in human actions?
(a) RASHI (17b, DH v'Chol she'Ken) explains that human actions are more likely to achieve perfect precision because they involve a concentrated effort to achieve an exact measure or timing. In contrast, natural occurrences involve no concentrated effort to achieve precision.
(b) TOSFOS (17b, DH Efshar) explains that it is possible to accurately measure the size or timing of a planned, human action. In contrast, a natural event occurs spontaneously, and thus it cannot be measured accurately.
According to Tosfos, even when natural events happen spontaneously, it is possible that there will be an exact measure. For example, it is possible for two animals to be born at the same time. However, one can never know for sure that they were born at once, because the timing of such a fleeting and unplanned event cannot be measured. (See also Rashi to 17b, DH Efshar l'Tzamtzem.)
2) RESOLVING A DOUBTFUL OWNERSHIP
OPINIONS: The Mishnah (17b) discusses a case in which a sheep gave birth to twin males, and it is not known which was born first. One of them dies before the owner gave one to the Kohen as the Bechor. Rebbi Tarfon maintains that the owner must share the living lamb with the Kohen. Rebbi Akiva maintains that the owner may keep the lamb until the Kohen brings proof that it is the firstborn (ha'Motzi me'Chaveiro Alav ha'Re'ayah).
The Gemara quotes Rebbi Chiya who says that Rebbi Tarfon's opinion can be compared to a case in which two people gave animals to a shepherd to tend. One of the animals died, and it is not known which one (RASHI DH l'Shenayim). The law is that the shepherd must place the living animal in front of the disputants and then be "Mistalek" -- remove himself, taking no further part in the dispute. In this case, neither owner may demand that the other one bring proof that the animal is his, because the animal is the physical possession of neither owner. Rashi (DH she'Meni'ach) explains that the Mishnah, in which Rebbi Tarfon says that the owner of the living lamb must split its value with the Kohen, similarly refers to a case in which the doubtful lamb had been deposited with a shepherd. Accordingly, both the owner and the Kohen must approach the shepherd, and neither one may demand proof of ownership from the other since the lamb is not in the possession of either one.
What is the meaning of "Mistalek" in the case in which one animal died in the possession of the shepherd?
(a) TOSFOS (DH she'Meni'ach) suggests that "Mistalek" means that the two disputants divide the remaining animal between themselves (like a ruling of "Yachloku"). Accordingly, the law is exactly the same as Rebbi Tarfon's ruling in the Mishnah (that the Yisrael and the Kohen divide the living lamb).
Alternatively, Tosfos explains that "Mistalek" means that the shepherd leaves and the principle of "Kol d'Alim Gavar" takes effect: whichever one of the two disputants is stronger is allowed to seize and keep the animal (see Insights to Bava Basra 35:1
Tosfos writes that if this is the meaning of "Mistalek," then the comparison between this case and the case of Rebbi Tarfon in the Mishnah is that in the same way that, in this case, one is not required to produce proof in order to receive the animal (since whoever is stronger prevails), in the Mishnah the Kohen is not required to produce proof in order to receive half of the living lamb.
The CHIDUSHEI REBBI AKIVA EIGER suggests that the two explanations in Tosfos depend on a dispute between the RIVA and RABEINU TAM cited by Tosfos in Bava Basra (34b, DH Hayu). The Gemara there states that when two people dispute the ownership of a ship anchored in the river, the Halachah is "Kol d'Alim Gavar." The Rishonim ask why the law in that case differs from the case of the Mishnah in the beginning of Bava Metzia (2a) in which two people are grasping a garment, in which case the Halachah is "Yachloku" (they divide it).
The Riva explains that in the case in Bava Metzia, it is not known for certain that one of the two disputants is a liar. It is possible that they both found and picked up the garment at the same time, and each one deserves half of it. In contrast, in the case of the boat on the river, one of the disputants is certainly a liar; the ship cannot belong to both of them. Therefore, the Halachah is not that they divide it (because this would not result in a just ruling), but rather that the stronger one prevails (and thus it is possible that the rightful owner will receive it; see Insights to Bava Basra 35:1
Rabeinu Tam explains that the difference between the case of the garment and the case of the boat is as follows. When a person is holding onto an object, his physical possession is considered proof that he has some ownership of it. Therefore, when two people are grasping a garment, each one's grasp indicates that he has ownership of the garment, and thus they split it. In the case of the boat, no one is holding onto the boat, and therefore they do not split it, but rather "Kol d'Alim Gavar."
In the case of the Gemara here, where two people deposited their sheep with a shepherd and one of the sheep died, there is no definite liar among the disputants, because no one knows for certain whose animal is the one that died. However, this case is also not a case in which the disputants both have possession of the object in doubt, because it is the shepherd who is holding onto the sheep. Therefore, according to the Riva, they should divide the sheep because there is no definite liar among the disputants, even though neither one has the sheep in his possession. This is the view followed by the first explanation in Tosfos.
According to Rabeinu Tam, since the sheep is not in the possession of either of the disputants, the law is "Kol d'Alim Gavar." This is the view followed by the second explanation in Tosfos. (D. BLOOM)
3) MAKING A "KINYAN" WITHOUT AN ACT OF "KINYAN"
QUESTION: The Gemara explains that Rebbi Tarfon and Rebbi Akiva disagree about the law in a case in which a Kohen is the shepherd for a Yisrael's animals on the Yisrael's premises. Rebbi Tarfon says that when the Yisrael's animal is about to give birth for the first time, he can be Makneh part of his Chatzer to the Kohen so that the Kohen will acquire the Bechor when it is born. RASHI (DH Aknuyei) explains that the owner of the animals does this because he wants the merit of having the Kohen use his land to raise the animals that belong to the Kohen (such as the Bechor).
The Gemara does not say that the owner or Kohen performs an actual Ma'aseh Kinyan, an act of acquisition, in order to transfer ownership of part of the Chatzer to the Kohen. How does the Kohen acquire the land without a Ma'aseh Kinyan? (TOSFOS DH Aknuyei)
(a) TOSFOS answers that a person is able to transfer his property to another person when he has full and unconditional intent, even without a Ma'aseh Kinyan.
The RASHASH adds that we find in a number of places that a Kinyan can take effect even without a Ma'aseh Kinyan, when the Makneh is receiving some form of benefit in return for transferring the ownership of his property. The Gemara (Gitin 14a and elsewhere) says, "In return for the benefit that he receives, he has full intention to transfer ownership of his property." Here, too, the owner of the animals is receiving the pleasure of helping the Kohen perform a Mitzvah by raising the Bechor.
(b) Tosfos answers further that the Kohen (the shepherd) frequently opens, and then locks, the door to the property of the owner. This action serves as an act of acquisition (Gitin 77b).
The Rashash, however, questions this answer. The Kohen is not aware that he is performing an act of Kinyan at the time that he opens or locks the door to the property. An act of Kinyan is not effective when done unknowingly (Yevamos 52b). How, then, can the Kohen acquire a portion of the Chatzer by unknowingly doing a Ma'aseh Kinyan?
The Rashash answers that the rule that a Ma'aseh Kinyan is not effective when done unknowingly applies only when the one performing the Ma'aseh Kinyan knows that there is an object present, but he does not know that the object is Hefker (and available to be acquired). When, however, he does not know that there is an object present (for example, the Kohen shepherd does not know that a Bechor was born in the Yisrael's herd), and had he known that the object was present he would have had intention to acquire it, he indeed is Koneh it even without his knowledge (TOSFOS to Bava Basra 54a, DH a'Da'ata).
The Rashash adds that according to the RA'AVAD (quoted by the NIMUKEI YOSEF to Bava Basra 41a), a Ma'aseh Kinyan done unknowingly works when an object is being given to a person. It is only when one is acquiring an object from Hefker that it does not work. The Makneh's intention suffices to transfer ownership of the object to the recipient (see Bava Metzia 11b, "Da'as Acheres Makneh").