On 23b R Papa says that in all three cases in the Gemara at the top of the page, there are witnesses - one witness who comes to say the opposite of the others. The Gemara asks why we need the Metziosa and why we need the Seifa.
The last question is answered by saying we might think that the woman who says her friend is tahor (in the reisha of the reisha) is believed on her friend because she made herself pasol and that she might not be believed if she says her friend is tahor when she says the she herself is tahor. Rashi says we might think she only wanted to benefit herself.
DO WE KNOW IN WHAT WAY THE WOMEN BELIEVES THAT SHE WILL BENEFIT HERSELF IF SHE SAYS HER FRIEND IS TAHOR?
David Morrison, Jerusalem, Israel
What Rav Papa is saying is that in the case where the woman claims that both she and her friend are Tahor, we would have thought that she is not believed even regarding her friend, because she only said that her friend is Tahor to get the Beis-Din to believe that she is Tahor too. That is what Rashi says. I see nothing about the woman benefiting herself, as you stated!
Be'Virchas Kol Tuv
Eliezer Chrysler