(Mishnah): False witnesses are only killed if the accused murderer had been sentenced (but not killed); in contrast to the Tzedukim (Sadducees) who said that they are only killed if the accused murderer had been killed, as they understood it from the pasuk (Devarim 19:21) - 'a soul for a soul'.

(Chachamim to the Tzedukim): But the pasuk already said (pasuk 19), "And you shall do to him as he intended to do to his brother" - meaning that his brother is still alive!

If so, why did the pasuk say 'a soul for a soul'? I might have thought that as soon as their testimony was accepted (even before the verdict, if they were found to be false) they can be killed; the pasuk teaches from 'a soul for a soul' that they are only killed after the verdict was made.

(Gemara - R. Zeira): From the Beraisa that follows, it is clear that a witness found to be false by Beis Din is not only disqualified as a witness from the time that he was found to be false onwards, but rather, it also applies retroactively back until the time that he originally gave the testimony. R. Zeira explained that the (upcoming) Beraisa refers to when they testified about two different things at the same time.

(Beraisa - R. Yosi): (If two witnesses testified that a person stole (an animal) and they also testified that he slaughtered it - if they were proved to be false about the theft, (since part of their testimony has been cancelled) their entire testimony is cancelled and the witnesses must pay double (as the punishment for the theft), but the accused is exempt for the extra payment for the slaughtering, since there is no longer testimony that he stole it.) When is it said (that the accused pays double, as the original testimony is not cancelled)? When they testified separately about the theft and the slaughtering; but if both testimonies were given at the same time, once part of the testimony is cancelled, the entire testimony is cancelled.

What is the meaning of 'if part of the testimony is cancelled, the entire testimony is cancelled'? On the 10th of Nisan they testified that the person stole an ox on the 1st of Nisan and that day (10th), he sold or slaughtered it. If they were then found to be false on the 15th of Nisan, all testimony that they gave from the 10th to the 15th is retroactively disqualified.

Rejection (R. Ba bar Mamal): (It could be that a false witness is disqualified from the point of discovery onwards, but here) R. Yosi was discussing when there was one testimony, so the testimony is fully cancelled at the same time - for example, when the theft and slaughtering were done at the same time; therefore, they are viewed as one occurrence and when part of it was proved false (such as the slaughtering) the entire testimony is disqualified. If so, this case is not a proof that a false witness is disqualified retroactively.

Support (Beraisa): When a pair of witnesses testified that Ploni was in possession of a field for three years and a second pair came and declared their testimony of the first year to be false, the entire testimony is now cancelled (as the beginning of the possession has been disproved). If they disproved the testimony of (only) the second year (so that the first pair are no longer believed even for the third year), all that remains is the testimony of the first year. If they disproved (only) the third year, the testimony of the first two years remains.

What is the case (of this Beraisa that we do not say that since the testimony of all three years came at the same time, as soon as the third year was disproven, the entire testimony falls)?

If they testified as one group, we have already discussed that their entire testimony would be cancelled.

Rather, it must be referring to when there was more than testimony - therefore, when the testimony of the second or third year was disproven, the testimony of the other years remain intact.

(Note: This sentence is removed by the Tosefos Rid.)