Makkos, Chapter 1, Mishnah 8. This mishnah continues the previous one and offers an additional explanation of why the Torah needed to mention a third witness, even though testimony is already valid with two witnesses.
The text of the mishnah:
"Keshem shesh'nei eidim, nimtza echad mehen karov o pasul - eidusan betelah" - just as with two witnesses, if one of them is found to be a relative or disqualified, their testimony is void. If one of the two turns out to be a relative of his fellow witness, or a relative of the person against whom they are testifying, or if he turns out to be disqualified from testifying, the entire testimony is void, since two witnesses no longer remain here.
"Af sheloshah, nimtza echad mehen karov o pasul - eidusan betelah" - so too with three: if one of them is found to be a relative or disqualified, their testimony is void. Even when there is a third witness, the disqualification of one of them disqualifies the entire testimony. We do not say: let us remove the disqualified one and two valid witnesses remain. Since they joined together as a single group of witnesses, all of them must be valid.
"Minayin afilu meah? Talmud lomar: eidim" - from where do we know that this applies even to a hundred? The Torah teaches by saying "witnesses." From where do we know that the law applies even to a group of a hundred? The word "eidim" (witnesses) teaches that their number makes no difference: they are all considered one group, and the law that applies to one of them applies to all of them.
The position of Rabbi Yosei:
Rabbi Yosei sets a limit on what is considered a single group of witnesses: "Bameh devarim amurim? Bedinei nefashos, aval bedinei mamonos - tiskayem ha'eidus bishar" - when does this apply? In capital cases, but in monetary cases the testimony stands with the remaining witnesses. Only in capital cases does the disqualification of one nullify the entire testimony, because in capital cases we always seek to be lenient, as the Torah says, "and the congregation shall save" - that they should seek to save the accused from death. But in monetary cases the testimony stands with the rest, that is, with the two valid witnesses who are not relatives.
The position of Rabbi:
Rabbi disagrees and rules: "Echad dinei mamonos ve'echad dinei nefashos" - both monetary cases and capital cases are the same. The disqualification found in one of the three or more disqualifies them all, and this law applies in monetary cases just as in capital cases.
However, in capital cases this applies only "bizman shehisru bahen" - when they warned them, that is, when the entire group of witnesses issued a warning to the one committing the transgression. "Aval bizman shelo hisru bahen - mah ya'asu shenei achim shera'u be'echad shehárag es hanefesh?" - but when they did not warn him, what should two brothers do who saw someone commit murder? In other words, if among the crowd of onlookers there happened to be two brothers, would the case be nullified and the killer never brought to justice? No. The witnesses are determined by who issued the warning to the transgressor - the warning is what defines them as witnesses, not the mere act of seeing.
Moreover, the Gemara explains that according to Rabbi, even in monetary cases, where there is no warning, they would ask the witnesses: at the moment you saw what happened, did you intend to serve as witnesses? Only one who intended to testify is considered a witness, and one who did not intend is not counted in the group and is not reckoned at all. But all those who did intend to be witnesses join together into a single group, and if one of them is found to be a relative or disqualified, that is a problem for all of them.
In summary: in this mishnah we learned that a group of witnesses is regarded as a single unit, and therefore the disqualification of one of them nullifies the entire testimony, even in a group of a hundred, as derived from the word "eidim." Rabbi Yosei limits this law to capital cases alone, based on "and the congregation shall save," while in monetary cases the testimony stands with the remaining witnesses. Rabbi applies the law to both areas, and defines the joining of witnesses into one group by the warning in capital cases, and by the intention to testify in monetary cases.