Taharos, Chapter 5, Mishnah 9. This chapter has been occupied with cases of doubt, sfeikos, in matters of tumah and taharah. Our mishnah takes up a doubt of a different flavor: not a doubt born of the circumstances, but a doubt born of people contradicting one another. One person insists that you contracted tumah, and you insist that you did not. Whose word carries the day?
Before we can enter the mishnah we need the standard the Torah sets for testimony. The pasuk teaches that a matter is legally established "al pi shnayim eidim", through the mouth of two, and it then adds "o al pi shelosha eidim yakum davar", or through three. If two already settle the question, what does the mention of three accomplish? It teaches that once the count of two has been reached, nothing further is gained: three, five or ten witnesses have precisely the legal force that two have, no more. A pair of eidim produces a verdict that cannot be shaken. A solitary eid stands on much weaker ground; there are areas of halachah in which his word is relied upon, and areas in which it carries no weight at all.
One witness against the person himself
"Eid omer nitmeisa v'hu omer lo nitmeisi, ne'eman": one witness says, "This man became tamei," and the man himself says, "I did not become tamei." He is believed and he remains tahor. This is one against one, and a lone eid does not have the strength to impose tumah on a man who denies it. The testimony is disregarded.
Two witnesses against the person himself
"Shnayim omrim nitmeisa v'hu omer lo nitmeisi": now two witnesses say he became tamei, and he denies it. Here the mishnah records a dispute. Rebbi Meir declares him tamei. His reasoning is straightforward: if the Torah trusts two witnesses even in capital cases, where a man's life hangs on their words, surely their testimony stands up against a man's own denial about a question of tumah.
The Chachamim say "hu ne'eman al yedei atzmo", he is believed concerning himself. The Bartenura points out that this is not to be understood as a person's word literally outweighing two kosher witnesses. Rather, we are able to read his statement in a way that does not clash with their testimony at all. How so? We interpret him as saying: "Certainly I became tamei, and these men are entirely correct about what they saw. But afterward I immersed in the mikveh, and I am now tahor." Since his words can be construed so that he is not contradicting the eidus but merely adding to it, we adopt that reading of his words, and he stands tahor.
Witnesses on both sides
The mishnah now turns to cases in which witnesses stand on both sides. "Eid omer nitma", a lone witness testifies that the man contracted tumah, "ushnayim omrim lo nitma", while a pair of witnesses testify that he did not. The ruling: "bein birshus hayachid bein birshus harabim, tahor", in a private domain and in a public domain alike, he is tahor. Where the incident occurred is irrelevant here, since nothing has been left unresolved: two witnesses have established that he remained clean, and their word decides the matter.
"Shnayim omrim nitma v'eid omer lo nitma": two testify that he became tamei and only one comes against them. Again "bein birshus hayachid bein birshus harabim, tamei", in either domain he is tamei, because we follow the two against the one.
One against one
"Eid omer nitma v'eid omer lo nitma, ishah omeres nitma v'ishah omeres lo nitma": a single witness against a single witness, or one woman testifying that he became tamei against another woman testifying that he did not. Now neither side of the argument is conclusive; each is a lone voice canceled out by the other. We are therefore left with a genuine safek, and a safek is decided by the standard rule of the domain in which it arose: "birshus hayachid, tamei; birshus harabim, tahor". In a private domain, a doubt of tumah is ruled tamei, so he is tamei. In a public domain, a doubt of tumah is ruled tahor, so he is tahor. That is the halachah.
With this the fifth perek draws to a close: testimony that is decisive settles the question outright, and testimony that is evenly matched hands the question back to the rules of safek.