1) TESTIMONY OF "CHATZI DAVAR"
QUESTION: The Mishnah states that when a different pair of witnesses testifies for each year of a Chazakah, the three testimonies of the three pairs combine and the Chazakah is established. The Gemara says that this does not follow the view of Rebbi Akiva who derives from the verse, "Al Pi Shnei Edim... Yakum Davar" (Devarim 19:15), that the entire matter ("Davar") must be made known to Beis Din by the two witnesses, and not half of the matter by two witnesses and half of the matter by another two ("Davar v'Lo Chatzi Davar"). How do the Rabanan, who accept the Chazakah established by the testimony of three different sets of witnesses testifying about three different years, understand this verse? The Gemara answers that according to the Rabanan, the verse teaches that "Chatzi Davar" in a different case is not accepted. In order to prove that a girl has reached maturity (and is liable for punishments), witnesses must testify that she has two hairs, even if one is on the back and one is on the front. When two witnesses testify that they saw a hair on the back, and another two witnesses testify that they saw a hair on the front, the Rabanan consider this "Chatzi Davar" and is not acceptable testimony.
What is the difference, according to the Rabanan, between the case of the Mishnah here and the case of the testimony about the hairs? In each case, there is a valid pair of witnesses testifying on part of a matter, and, when combined, their testimony attests to a full Chazakah or to a full sign of maturity!
(a) The RASHBAM explains that the difference is that in the case of the hairs, neither pair of witnesses saw everything that was possible to see at the time. Had the witnesses checked more extensively, they would have seen the hair in the other place as well. Therefore, their testimony about one hair is considered testimony on an incomplete matter, and it cannot combine with the other pair's testimony on an incomplete matter. In contrast, in the case of a Chazakah, each pair of witnesses saw everything that was possible to see at that moment -- the pair witnessed the Machzik occupying the field for a year. During that year, it was impossible, obviously, for that pair of witnesses to see the Machzik occupying the field for any other year. Therefore, their testimony about that year is considered testimony on a complete matter. This is also the explanation that the BA'AL HA'ME'OR here prefers. Tosfos in Bava Kama (70b) gives this explanation in the name of the RI.
The KOVETZ SHI'URIM (#255) explains that the Rashbam understands the Pesul of "Chatzi Davar" to be a lack not in what the witnesses say ("Hagadas ha'Edus"), but in what the witnesses see ("Re'iyas ha'Edus"). That is, at the moment that they saw the matter, were they seeing only half of what there was to see (as in the case of the hairs), or were they seeing everything that there was to see (as in the case of the Chazakah)? It is not a Pesul in what they say to Beis Din (that is, two witnesses say that they saw one part of the Chazakah, and another two witnesses say that they saw another part of the Chazakah).
(b) TOSFOS (DH Ela) cites RASHBA (Rabeinu Shimshon mi'Shantz) who explains that the difference is that testimony of two witnesses about the presence of one hair has absolutely no ramifications at all. Without testimony about the second hair, the testimony is completely irrelevant and useless. In contrast, testimony about a single year of occupancy of a field is considered a complete and valid testimony in itself for a different matter -- to prove that the Machzik ate the produce of the field and must reimburse the owner of the field (if no Chazakah is established) for a year's worth of produce. This is also the explanation of the RIF here.
The CHIDUSHEI RABEINU MEIR SIMCHAH explains that Tosfos, too, understands that the Pesul of "Chatzi Davar" is a Pesul in the "Re'iyah" of the witnesses, in what they saw, and not in what they say. Hence, if, at the time that they see the event, what they saw is relevant in itself for a complete matter, then it is considered a valid testimony that can combine with the testimony of other witnesses. (See, however, the CHASAM SOFER's explanation of the RIF.)
(c) The RI MI'GASH and the RIF (in Teshuvos) explains that the reason why the witnesses do not combine in the case of the hairs is that the Halachah is that the two hairs must be in the same place. This is the subject of a Machlokes Tana'im in Nidah (52b), and the Halachah follows the view that the two hairs must be in one place. Therefore, even if one pair of witnesses testifies about two hairs, one on the back and one on the front, that testimony is not valid.
The Rishonim (BA'AL HA'ME'OR, RAMAH, RASHBA) reject this explanation because it gives an entirely different reason for why the testimony of the witnesses in the case of the hairs is not valid. It is not because of "Chatzi Davar" but because of a specific requirement in the location of the hairs that has not been fulfilled. Accordingly, this explanation is not consistent with the words of the Gemara here, which says that the reason why the testimony is not accepted is that it is "Chatzi Davar."
(d) RASHI in Bava Kama (70b) writes that in the case of the testimony about a girl's hairs, according to the testimony of each witness the girl is still a Ketanah and therefore their testimonies do not combine, while in the case of a Chazakah, according to the testimony of each witness the Machzik is occupying the field. The RAMBAN and RASHBA explain that Rashi means that when two witnesses testify about one hair, that in no way indicates anything about the girl's maturity, since even a very young girl has a single hair. Therefore, that "Chatzi Davar" is not acceptable testimony. In contrast, when two witnesses testify that they saw the Machzik occupying the field for one year, they are testifying about the Chazakah itself -- that the Machzik acted in a way that demonstrated his ownership of the field. Even though further testimony is needed to tell Beis Din that the field did not leave the Machzik's possession during the next year, nevertheless if it is assumed that the facts provided by the testimony about the first year remained unchanged (i.e. the Machzik continued to occupy the field), then the Machzik would have a Chazakah. Hence, the testimony about the first year is the beginning of the proof about the subsequent years, and thus the testimonies about the different years combine. (See AVI EZRI, Hilchos Edus 21:9.)
The RAN explains Rashi differently. The Ran explains the mechanism of a three-year Chazakah like the RAMBAN (see Insights to Bava Basra 29:2): even when the Machzik occupies a field for one day without the protest of the original owner, that is enough to establish proof of his ownership. The fact that the previous owner did not protest immediately proves that he is not the true owner. A true owner would immediately object to someone else using his land. However, the fact that the Machzik cannot show a Shtar weakens the Machzik's claim of ownership. The weakened Chezkas Mara Kama overrides the weakened Chazakah of the Machzik. Only after three years have passed is the claim of the Machzik 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. Accordingly, when the first two witnesses testify that the Machzik occupied the field for one year, that suffices to establish a Chazakah of his ownership. The other two pairs of witnesses, who testify about the second and third years, are necessary only to remove the element that weakens the Machzik's claim.