1) LAND OWNED BY "BEI BAR ELYASHIV"
QUESTION: The Gemara explains the source for the "Chezkas Shalosh Shanim," the Chazakah of three years, according to the Chachamim. The Gemara first suggests that the true owner is disturbed (Makpid) only when someone occupies his field for three years, but not when someone occupies his field for less than that time. That is why it takes three years of no protest from the previous owner in order to make a Chazakah. Abaye asks that if this is true, then when someone uses a field that belongs to Bei Bar Elyashiv, who protest immediately even if someone merely passes briefly over their borders, the person should have a Chazakah immediately if the owner did not protest. However, the Halachah makes no distinction with regard to the time it takes to make a Chazakah; it takes three years to make a Chazakah, regardless of the degree of Hakpadah of the owner of the field.
Why does the Gemara not answer simply that when the Chachamim establish a Shi'ur for something, that Shi'ur is an amount that applies to all circumstances without exception (see Gemara 23b)? (RASHBA)
ANSWER: The RITVA answers that the Gemara is not asking merely from a case of landowners like Bei Bar Elyashiv. The Gemara means that there is no rationale for making three years the standard time that it takes a person to protest, since most people will protest in less than three years. (See CHIDUSHEI RABEINU MEIR SIMCHAH.)
(The Gemara answers that a person normally keeps his Shtar for three years because he suspects that the previous owner will take up to three years to protest, or he will claim that he is the type of person that takes a long time to protest. See following Insight.)
2) KEEPING A "SHTAR" FOR THREE YEARS
QUESTIONS: The Gemara explains the source for the "Chezkas Shalosh Shanim," the Chazakah of three years, according to the Chachamim. The Gemara first suggests that the true owner is disturbed (Makpid) only when someone occupies his field for three years, but not when someone occupies his field for less than that time. That is why it takes three years of no protest from the previous owner in order to make a Chazakah. Abaye asks that if this is true, then when someone uses a field that belongs to Bei Bar Elyashiv, who protest immediately even if someone merely passes briefly over their borders, the person should have a Chazakah immediately if the owner did not protest. However, the Halachah makes no distinction with regard to the time it takes to make a Chazakah; it takes three years to make a Chazakah, regardless of the degree of Hakpadah of the owner of the field.
Rava concludes, therefore, that the reason why a Chazakah is made in three years is that a person normally keeps his Shtar for only three years and not more. Therefore, after three years it is no longer necessary for him to present a Shtar in order to support his claim of ownership of the land.
There are a number of questions on this conclusion.
(a) Rava explains only why the lack of a Shtar, after three years have passed, cannot prove that the person on the field is not the owner of the field. However, Rava does not explain what proves that he is the owner of the field. Consequently, the field should be given back to the previous owner, who has proof that he once owned the field (and thus has a Chezkas Mara Kama). (Rishonim)
(b) If the logic of Chazakah is based only on the fact that a person does not keep his documents for more than three years, then what is the meaning of the Gemara that follows, that teaches that in a place where people sow their fields every other year, a person must occupy the field for six years in order to make a Chazakah? Moreover, the Gemara later (36b) teaches that even the Chachamim who argue with Rebbi Yishmael agree that a person can make a Chazakah in less than three years on a "Dekel Na'arah," a young tree which bears three crops in less than three full years. If a person normally keeps his Shtar for three years, then why should he have a Chazakah in less time? (See RASHBAM DH b'Asra.)
ANSWERS:
(a) The Rishonim suggest a number of answers to this question.
1. The RASHBA explains that if the previous owner did not complain for three years, then Beis Din must suspect that he waited three years in order for the Machzik to stop saving his Shtar so that his claim would be effective. This suspicion weakens his Chezkas Mara Kama, and therefore the claim of the Machzik (who is presently on the land) is stronger than the Chezkas Mara Kama.
2. The RAMBAN (42a) and RITVA explain that even in less than three years the fact that the previous owner did not protest immediately is proof that he is not the true owner (as the Rashba says with regard to one who does not protest for three years). A true owner would object immediately when someone occupies his land. However, the fact that the Machzik cannot show a Shtar weakens the Machzik's claim as well. The weakened Chezkas Mara Kama overrides the weakened Chazakah of the Machzik. After three years, on the other hand, the claim of the Machzik is no longer weakened by his lack of possession of a Shtar, since he is not expected to keep a Shtar for that long, and therefore his Chazakah prevails.
(c) TOSFOS in Bava Metzia (110a, DH Amar Lei) and the TOSFOS HA'ROSH there explain that the "Chezkas Shalosh Shanim" is only a Takanah d'Rabanan. Since the Rabanan found that people normally do not keep their Shtaros for more than three years, they instituted that if the previous owner wants to protest when another person uses his field, he must do so within three years, before the Machzik loses his Shtar (if he has one).
This also seems to be the intention of TOSFOS here in Bava Basra (35b, DH v'Iy l'Peira), the ROSH, and the NIMUKEI YOSEF here, as well as the KETZOS HA'CHOSHEN (140:2).
Does this mean that the proof of ownership that comes from a Chazakah is only mid'Rabanan and relies on the principle of "Hefker Beis Din Hefker"? The SHA'AR HA'MISHPAT (CM 140) writes that once the Rabanan instituted that a person must protest within three years if the field is his, it becomes a valid proof d'Oraisa -- when he does not protest within that time -- that the Machzik is the owner (as the Rasha writes, as cited above).
(b) The RASHBA explains that even according to Rava's conclusion, a Chazakah depends on the amount of time that it takes for the original owner to be disturbed that someone is on his field. Since the owner will protest the presence of a stranger on his field only after three crops have been harvested from the field, in a place where fields are left fallow every other year the owner will have six years within which to protest. Only after six years pass will the Machzik not keep his Shtar anymore, because he is confident that the previous owner will no longer make a claim on the field.
Similarly, when a person is Machzik a "Dekel Na'arah" he will not keep the Shtar after three crops have been harvested, because if the previous owner intended to protest, he would have protested before the time that it takes the tree to produce three crops.
(This explanation can be used according to all of the explanations mentioned above for how a Chazakah proves ownership.)
According to this explanation, how does Rava answer the question from the case of Bei Bar Elyashiv? The Machzik should assume that he does not need to keep the Shtar if Bei Bar Elyashiv does not protest immediately, and he should have a Chazakah immediately!
The Rashba explains that since the Chazakah is determined by the Machzik's expectation of when the owner should protest (and not by Beis Din's assessment of when the owner should protest) -- a Chazakah needs three years even on the land of Bei Bar Elyashiv. The reason for this is that the Machzik will always keep the Shtar for three years because of his fear that the previous owner will claim that he is not the type of owner who protests in less than three years. (See also NESIVOS HA'MISHPAT CM 140.)
Support for this answer may be found in the Gemara later (57b) which says explicitly that a Chazakah depends on whether or not the previous owner would protest such an action on his property. (See Rashba to 57b who seems to take a different approach to this question.)
3) LEAVING LAND FALLOW WHEN MAKING A "CHAZAKAH"
QUESTION: The Gemara says that in a place where people normally leave land fallow every other year, a Chazakah can be made in six years (and the lack of use of the land for the alternating three years does not ruin the Chazakah). The Gemara asks that this is obvious, since it is the normal manner to work the land in that place by leaving it fallow every other year. The Gemara answers that this teaching is necessary in a place where some people do not leave their land fallow, and other people do; in such a place, if the Machzik leaves the land fallow he still has a Chazakah. One might have thought that he does not have a Chazakah since the previous owner can claim that if the land truly belongs to the Machzik, then he should have sown it.
Why should the Machzik have sown the land? If some people in that place leave the land fallow, why would he be expected to be among those who do not leave their land fallow?
ANSWER: The NIMUKEI YOSEF answers that since the field is newly acquired (according to the Machzik's claim), the Machzik should have planted it three years consecutively in order to hasten the establishment of his Chazakah.

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