According to Rava's first suggestion, the Rabbanan's source for the Chazakah of Karka is the S'vara of Mechilah. What he means is - we assume that Reuven is Mochel Shimon, when he works on his field and eats the Peiros up to two years, but not three. Consequently, his failure to protest during that third year substantiates Shimon's claim that he sold him the field.

Abaye rejects ...

... this on the grounds - that if it were so, then when Shimon returns the field, he ought to be exempt from paying Reuven for the fruit that he ate, yet Rav Nachman obligates him to pay for the fruit as well.

... Rava's second suggestion that for the first two years Reuven does not object to Shimon's working on his field (although he is not Mochel), whereas in the third, he does object on the grounds that - in that case, a Chazakah on the property of people who tend to be fussy, and would never allow anyone to work on their land under any circumstances, such as the bei bar Elyashiv, should be effective immediately.

That cannot be the case however - due to the principle of 'Nasata Devarecha le'Shi'urin' (Chazal's Takanos need to be uniform, and cannot be tailored to suit each person's individual needs).

Rava's final S'vara to explain the three-year Chazakah of Karka according to the Rabbanan is that - Reuven needs to warn Shimon within the three-year period, because the latter tends to look after his Sh'tar up to that time and no longer. The purpose of the warning therefore, is to deprive him of an excuse to later claim that he had a Sh'tar but lost it.

Abaye asked Rava why, in that case, a Mecha'ah not in the presence of the Machzik, should be effective. Rava replied that - the latter cannot argue that had the Mecha'ah been made in his presence, he would have made sure not to lose his Sh'tar, because based on the S'vara that we cited earlier 'Chavrech Chavra is leih ... ', we assume that he certainly heard about the Mecha'ah, in which case he ought to have looked after his Sh'tar anyway.

Rav Huna requires a Chazakah to last for three consecutive years. Otherwise, we might have interpreted our Mishnah's statement 'Shalosh Shanim mi'Yom el Yom' to mean that - the years must be complete (to preclude 'Shanim Mekuta'os of the Seifa), even if they are not consecutive.

According to Rav Chama, Rav Huna concedes that the Chazakah takes effect even if the three years are not consecutive 'be'Asra de'Movri Bagi', meaning - in a place where it is customary to work the land only every alternate year.

To answer the Kashya why that is not obvious, we establish the case - where the Minhag is not absolute, but that some people tend to leave their fields fallow and others do not, and he is teaching us the even if Reuven finds Shimon not working the land one year, he will nevertheless need to make a Mecha'ah within the six year period (if he wants to stop Shimon from making a Chazakah).

The reason Reuven cannot counter that Shimon ought to have worked the field for three consecutive years (since not everyone uses the rotation system anyway)is - because Shimon can reply that, seeing as all the other residents of his particular valley left their lands fallow every second year, he found it uneconomical to employ a guard for his field only, during those years.

Even if they do not however, the Chazakah will still take effect, because Shimon can argue that - irrespective of what others do, as far as he is concerned, the rotation system enables his field to yield a more bountiful harvest during those years that he worked the land.

If, as Rav Huna asserts, the Chazakah must be unbroken - how will anyone ever be able to make a Chazakah on a house, since what will Shimon do should Reuven claim that he came on certain nights and discovered that he was not sleeping there?

Abaye answers this Kashya however - by pointing to the neighbors, who know exactly what goes on in Reuven's house, and will be able to testify that Shimon was there every night.

Rava establishes the Chazakah on houses when Levi and Yehudah testify that they hired the house from Shimon and lived there for three years, day and night. The problem Rav Yeimar has with this is - the fact that Levi and Yehudah themselves are prejudiced inasmuch, as if they do not make this testimony, then Reuven will take the house back from them.

Rav Ashi answered Rav Yeimar's Kashya by explaining that - the Tana is speaking where Levi and Yehudah have not yet paid, and are prepared to pay the owner, whoever he is (so it makes no difference to them whether Reuven ends up taking the house or Shimon).

Mar Zutra rules that a case where Reuven demands that Shimon brings two witnesses to prove that he lived in the house day and night - that Reuven is acting within his rights when he demands that Shimon brings two witnesses to support his claim.

The point of ...

... Mar Zutra's statement is (not merely to repeat what Rav Ashi just said but) to teach us that, even though Shimon requires three full unbroken years for a Chazakah, he is not generally expected to bring proof of use for every day and night during that period - should Reuven demand it, then he is obligated to do so.

... Reuven's demand is to justify - his failure to make a Mecha'ah.

Rav Huna will agree that we force Shimon to prove every day and night that he stayed in the house, even though Reuven did not demand it personally - if Reuven is a peddler, who travels from town to town, and who is therefore unable to demand it of Shimon, simply because he is not always in town, and cannot keep a check on Shimon's comings and goings. (Note however, that a Mecha'ah of this nature requires Reuven's positive knowledge that Shimon was not there during the time that he demands proof).

On the other hand, Rav Huna will concede that even if Reuven does ask for proof that Shimon lived in the house day and night, we ignore his demand, if it concerns the stores of Mechuza - because the bakeries and wine-stores of Mechuza were open by day but closed by night, and since it was common to limit their use to the day, Reuven's demand for Shimon to prove that he used it at night is not justified.

Rami bar Chama and his brother Mar Ukva bar Chama entered into an agreement - that they would take turns in working with the Shifchah that they had purchased jointly, on an annual basis.

Despite the fact that they had already owned her for three years, when a third party claimed ownership of the Shifchah - Rava ruled that just as their agreement prevented them from making a Chazakah against each other, so too, did it prevent them from making a Chazakah against any third party.

When he added that, had they written 'It'da' (or 'Itra') it would have had a Kol - he meant that writing their condition into a Sh'tar would have enabled them to make a joint Chazakah against a third party ...

... because whatever is written in a Sh'tar becomes publicized, in which case the claimant ought to have been warned to make a Mecha'ah.

The Chazakah would then take effect after three years - to conform to the ruling later that if three purchasers buy a field one from the other in three consecutive years, then, provided each one has a Sh'tar (It'da), they combine to make a Chazakah, because if Reuven claims that he did not bother to make a Mecha'ah (because, seeing them taking turns, he thought they were afraid to make a proper Chazakah), the Kol produced by the Sh'tar It'da, proves this claim to be false.

Rava rules that if Shimon makes a Chazakah on all but a Beis Rova of Reuven's field - he acquires the entire field except for that Beis Rova.

Rav Huna b'rei de'Rav Yehoshua restricts the Beis Rova - to where it is fit to produce crops; otherwise, Shimon will acquire even it together with the rest of the field.

Rav Bibi bar Abaye rules - that one can make a Chazakah on a piece of rock (even though it is not arable) by placing one's animals there, or spreading out fruit.

Based on this ruling, he refutes Rav Huna b'rei de'Rav Yehoshua's qualification inasmuch as - since Shimon could have acquired the non-arable Beis Rova in either of these two ways, but failed to do so, there is no reason for him to acquire it together with the rest of the field.

When, in a case where Shimon proves that he made the necessary Chazakah on his house, Reuven counters 'Ana bi'Shechuni Gava'i Hava'i', he means - that he did not bother to make a Mecha'ah because, throughout the three years that Shimon was living in the house, he was living there in the inner rooms.

Rava objected to Rav Nachman's ruling ordering Shimon to prove that he lived alone in the house for three years, on the grounds that - since Shimon completed his Chazakah according to the Halachic requirements (without any sign of a hitch), he was a Muchzak, and the onus lay on Reuven to prove that he lived in the inner rooms.

This Machlokes must be speaking where there were independent witnesses to the effect that the house had previously belonged to Reuven, because otherwise - Rav Nachman would have conceded that Shimon is believed, on account of the principle 'Peh she'Asar (Shimon, without whose admission we would not know that the house had belonged to Reuven), Hu ha'Peh she'Hitir' (when he added that he bought it from him).

We would have then believed Shimon when he claimed that he bought the house from Reuven - because he could have said that the house had always belonged to him, in which case, he would have been believed.