Mishnah 8. In the previous mishnah we learned that a single witness is believed to testify that a man is a kohen, and on his word we allow him to eat Terumah. In our mishnah Rabbi Yehudah disagrees.
Rabbi Yehudah's position:
Rabbi Yehudah holds "ein ma'alin li'kehunah al pi ed echad" - a single witness is not believed, and two witnesses are required in order for a man to be considered a kohen and to be fed Terumah. This is the law even where there is no concern of collusion as there was in the previous mishnah, where one might have arranged with his fellow that each would testify for the other. Even without that concern, in his view a single witness is not believed.
Rabbi Elazar's ruling:
Rabbi Elazar qualifies this: when is a single witness not believed? Specifically "bi'makom she'yesh or'rin" - when there are people who come and contest the kohen's lineage, claiming that he is disqualified from the priesthood. But where there are no challengers, where no one claims that he is disqualified, and there is also no concern of collusion but rather an independent witness comes and testifies that this man is a kohen - "ma'alin li'kehunah al pi ed echad". So Rabbi Elazar disagrees with Rabbi Yehudah, and holds that we can rely on a single witness who testifies that a man is a kohen and feed him Terumah.
The position of Rabbi Shimon ben HaSegan:
The mishnah cites in his name that we do elevate a man to the priesthood on the word of a single witness, and the Gemara explains that he means even in a case where there are challengers - even when there are those who claim that he is disqualified.
The case in the Gemara:
A man whose father was known to be a kohen, and a report (rumor) circulated about him that he is the son of a divorcee and therefore disqualified from the priesthood.
A single witness came and testified that he is a valid kohen - and according to all opinions a single witness nullifies a mere rumor, since there is nothing here beyond hearsay.
Two witnesses came and testified that he is the son of a divorcee and disqualified - and with that he is disqualified, since we have the testimony of two.
Another lone witness came and testified that he is a valid kohen.
Rabban Gamliel's position:
Rabban Gamliel holds that in the final reckoning we have here two against two: the first witness, who nullified the rumor, and the last witness join together, and against them stand the two witnesses who testified that he is disqualified. And since he originally had a presumption of priesthood, because his father was a kohen, we combine the two individual witnesses into a single testimony and leave him with his presumptive status: on the strength of his father having been a kohen, he too is presumed to be a kohen.
Rabbi Elazar's position in this case:
Rabbi Elazar, who holds that where there are challengers we do not consider him a kohen, would disagree: we do not combine the two individual witnesses with each other to create two against two. Consequently we follow the two witnesses who came together and testified that he is not a kohen, and on their word it is established that he is not a kohen.
In summary: This mishnah records a dispute about the credibility of a single witness to establish a man's priesthood - Rabbi Yehudah rejects a single witness and requires two; Rabbi Elazar distinguishes between a case where there are challengers and a case where there are none; and Rabbi Shimon ben HaSegan accepts a single witness even where there are challengers. Following this we examined the case in the Gemara - a rumor, a single witness who nullifies it, two witnesses who disqualify him, and one further lone witness - and the dispute between Rabban Gamliel and Rabbi Elazar as to whether we combine the two individual witnesses into two against two and leave the man with the presumption of his father's priesthood.