1) THE MECHANISM OF THE HEFKER OF SHEMITAH
OPINIONS: The Gemara defines the word "Netushim" (which is used by the Tosefta cited on 38b) as landowners who abandoned their land against their will. The word is derived from the verse, "veha'Shevi'is Tishmetenah u'Netashtah" (Shemos 23:11), in which "u'Netashtah" indicates that the fruit of one's land automatically becomes Hefker in the Shemitah year, regardless of whether the owner wants it to become Hefker.
The Mefarshim discuss exactly how the fruit of one's land becomes Hefker at the arrival of the Shemitah year.
(a) The MAHARIT (1:43, in the name of his father, the MABIT) proves from this Sugya and from other places that the fruits of one's field indeed become Hefker against his will. The MINCHAS CHINUCH (#84) writes that if the owner does not "make" his fruit Hefker, it becomes Hefker anyway, and his refusal to let others into his field is more than a mere lack of fulfillment of the Mitzvas Aseh to make his field Hefker; it constitutes an act of Gezeilah from the public (since the fruits automatically become Hefker and the public has the right to take them).
(b) The BEIS YOSEF (cited by the Maharit 1:44), however, maintains that the fruit of the field does not become Hefker unless the owner makes it Hefker. Nevertheless, the Gemara here calls the fruit "an appropriation of the King" ("Afka'asa d'Malka") because the Torah commands the owner to make it Hefker. (I. Alsheich)
2) CONCERN FOR THE POSSIBILITY OF DEATH
QUESTION: Rav Huna (39a) rules that Beis Din may not appoint a minor to manage the estate of a relative who has been taken captive (or has involuntarily abandoned his land in some other manner), because of the concern that the minor may not take proper care of the land. Rav Huna also rules that Beis Din may not appoint a relative to manage the estate of a minor, lest the relative claim that the estate fell to him, and not to the minor, as an inheritance (and the minor will not be able to make a counter-claim).
The Gemara demonstrates the application of these rules with an actual incident that occurred. An elderly woman had three daughters, one of whom had a son who was a minor. One daughter, the mother of the child, died. The elderly woman and one of her daughters were taken captive, leaving one daughter and one grandson. Abaye questioned what should be done with the land of the elderly woman and the daughter who were taken captive. Normally, it would be entrusted to the care of the remaining relatives. In this case, however, it could not be given to the child because of Rav Huna's ruling that Beis Din may not appoint a minor to manage the estate of a relative in captivity. On the other hand, the land could not be placed in the care of the remaining sister, because perhaps the elderly woman had died, leaving her land to her three daughters, and the child inherited his mother's portion of the land. In such a case, Rav Huna rules that Beis Din may not appoint a relative to manage the estate of a minor.
Why was Abaye concerned that the elderly woman may have died? The Gemara in Gitin (28a) teaches that the Rabanan are never concerned with the possibility that a person may have died, because there is always a Chazakah that a person is alive (that is, since he was alive until now, it is assumed that he is still alive until proof is brought that he died). This Chazakah applies even to an old person; the Mishnah in Gitin there teaches that if an elderly man sends a Shali'ach to give a Get to his wife, the Shali'ach may give the Get and need not be concerned that the man may have died. Why, then, is the Gemara here concerned that the elderly woman may have died?
(a) TOSFOS and TOSFOS RABEINU PERETZ answer that the elderly woman in this incident had already reached the age of "Gevurah" (over the age of 80; see Moed Katan 28a, and Avos 5:27). The Gemara in Gitin says that there is no longer a Chazakah that a person is alive once that person has reached the age of Gevurah.
However, Tosfos and Tosfos Rabeinu Peretz question this answer based on Abaye's ruling in this incident. Abaye ruled that the remaining sister should receive half of her elderly mother's land (and the other half should be given to an Apotropos and not to the child), because if the elderly woman and her daughter had both died, then the remaining sister and the grandson would split the estate. Based on the Gemara in Gitin, however, the remaining sister should have received two thirds; she should have received the amount (one third) that she might have inherited from her mother if her mother died, and she should also be placed in charge of the portion of her sister in captivity, which is also one third of the land. Her sister certainly would be assumed to be alive, since she certainly did not reach the age of Gevurah yet. If the sister had reached the age of Gevurah, then the mother would have reached that age long before, and the Gemara in Gitin says that once a person has reached the age of Gevurah and has continued to live for a long time, Beis Din again disregards the possibility that the person may have died. Thus, if the sister reached the age of Gevurah and there is no Chazakah that she is still alive, there would be no reason to suspect that her mother died. It thus would be impossible to suspect that both the daughter and the mother had died.
Tosfos rejects his initial answer because of this problem. Tosfos Rabeinu Peretz, however, defends it (as does the RITVA) and argues that a person who has passed the age of Gevurah can be assumed to be alive only when that person has reached the age of 100. In this incident, it was possible that the elderly mother had reached the age of Gevurah but had not reached 100 (for example, she was 97), and her daughter had reached Gevurah as well (for example, she was 80). Therefore, it is logical that Abaye was concerned that they both may have died. (The RAMBAN in Gitin says about this answer that "Lav Milsa Hi," it is not a valid answer. Tosfos here does not give this answer presumably because he sees no reason to differentiate between one who has lived passed the age of Gevurah by just a few years, and one who has lived passed Gevurah for twenty years. See also RASHI in Gitin there who writes that "one who is 81 or more, until ninety, is close to death," implying that once a person has passed the age of ninety he is assumed to be alive and the Chachamim do not suspect that he has died. See also the TOSFOS RID there.)
(b) In another answer, TOSFOS explains that only with regard to giving a Get do the Chachamim apply the Chazakah that a person is still alive even to an old person. In the case of a Get, there is a concern that the woman will be left an Agunah, because no one will know whether or not her husband died, and the Chachamim therefore were lenient and allowed the woman to become divorced on the assumption that her husband is still alive at the time the Get is given. Similarly, a Kohen may eat Terumah or the meat of a Korban without concern that an immediate relative of his may have died (even though he would then be forbidden to eat Kodshim). In contrast, in the case of questionable ownership of land, the Chachamim are stringent with regard to determining the most appropriate and responsible course of action to take with the estate. In order to protect the interests of the heirs who are minors, the Chachamim do take into account the possibility that the captives may have died. This is why Abaye did not appoint the relative as manager over the estate that the minor might have inherited. (See TERUMAS HA'DESHEN (#349, "Amnam Ki Daikinan") for further explanation of this answer of Tosfos.)
Tosfos gives an alternative reason for why, in this case, Abaye was stringent to take into consideration the possibility that a person may have died. Tosfos says that since the elderly woman and her daughter were taken captive, there was a strong reason to assume that they had died, since captors tend to afflict their captives and cause them to die.
(c) The RAMBAN in Gitin (as cited by the RITVA here), the CHIDUSHEI HA'RAN in Gitin, and RABEINU KRESKAS answer that when the property of a minor is being held by Beis Din, the members of Beis Din are obligated to do whatever necessary -- and to take into consideration any possibility -- in order to ensure that their decision is proper and just. Therefore, Beis Din must take into account the possibility that the captives may have died. In contrast, in the case in Gitin there is no need to take into account all of the possibilities, since that case does not involve a ruling of Beis Din, and the one giving the Get therefore may rely on the Chazakah that the person is still alive. The Ramban in Gitin explains further that in the case of the Gemara here, Beis Din has a clear course of action to take with regard to the land, even when taking into account the possibility that the captives may have died. In Gitin, however, if the Shali'ach were to be concerned that the man may have died, then he would have no appropriate course of action to take, and the woman would be left as an Agunah. (I. Alsheich)