(Mishnah - R. Chanina Segan ha'Kohanim): I never saw a skin taken to Beis ha'Sereifah (to be burned due to a Pesul).

R. Akiva: From R. Chanina we learn that (even) if a Bechor was found to be Treifah after flaying, Kohanim receive the skin.

Chachamim: Not seeing is not a proof! (Perhaps a Pesul never occurred.) The Halachah is, it goes to Beis ha'Sereifah.

Eruvin 41a (Beraisa): After R. Gamliel died, R. Yehoshua came to overturn his words (and say that if Tish'ah b'Av falls on Erev Shabbos, we complete the fast).

R. Yochanan ben Nuri: As long as R. Gamliel was alive, the Halachah followed him. We will not annul his words now. The Halachah was fixed like him!

No one argued with R. Yochanan ben Nuri.

Pesachim 100a (Beraisa): Once, R. Shimon (ben Gamliel), R. Yehudah and R. Yosi were eating, and Shabbos came. R. Shimon suggested to R. Yosi that they should be concerned for R. Yehudah's opinion (and interrupt).

R. Yosi: You always try to get R. Yehudah to agree with my opinion. Why now do you try to get me to agree to his opinion?!

R. Shimon: If so, we will not interrupt, lest Talmidim see and the Halachah will be fixed like R. Yehudah.

Kidushin 50b (Rav Papa): In an area where people are Mekadesh and later send Sivlonos (prenuptial gifts), we are concerned (lest he already was Mekadesh her). In an area where people are Mekadesh after Sivlonos, we are not concerned.

Rashi (103b DH Ein): Perhaps a Tereifah never occurred after flaying in the days of R. Chanina, and even if it happened and it was burned, perhaps he did not see it.

Maharik (171): When do we say that not seeing is not a proof and not hearing is not a proof? If two pairs of witnesses contradict each other, i.e. one pair says that they saw Kidushin, and the other says that they did not see. However, when there is no contradiction, e.g. if no one testifies that the custom was to be stringent about Sivlonos, surely we rely on those who testify and say that they never saw or heard anyone (be stringent). In Eruvin, we did not say that everyone agreed with R. Yochanan ben Nuri, rather, that no one argued with him. We did not say that not seeing is not a proof!

Shiltei ha'Giborim (Bava Metzia 52a): We find that R. Shimon ben Gamliel wanted to interrupt, like R. Yehudah, but after R. Yosi protested, he retracted, lest Talmidim see and the Halachah will be established like R. Yehudah. Without a protest, Talmidim would not think that they interrupted because the Halachah requires this. Rather, it just occurred this way. Not seeing is not a proof, like it says in Zevachim. Rashi explains that perhaps R. Chanina never saw a skin taken to Beis ha'Sereifah, for a Pesul never occurred in his days. Or, perhaps a Pesul occurred, but he did not see it.

Rebuttal (Shach CM 37:38): There is no proof from Pesachim. Surely the Halachah would not be fixed for generations based on one episode unless there was a protest. However, if it happened many times and there was a custom, surely they could learn from the fact that it never happened oppositely. Surely, a custom is a proof. There is no proof from Zevachim. Rashi says that perhaps a Tereifah never occurred after flaying in the days of R. Chanina, and even if it happened and it was burned, perhaps he did not see it. I.e. it is no proof because it is not common to be found Tereifah after flaying. Also, perhaps he did not see, but others did.

Shiltei ha'Giborim (ibid.): Maharik says that not seeing or hearing is a proof regarding Sivlonos, from the case in which they fixed the Halachah like R. Gamliel because no one protested. This is because they testified about a custom established like R. Gamliel.

Rebuttal (Shach ibid.): Shiltei ha'Giborim says that not finishing the fast was a custom. If Maharik meant this, what was his proof from there to Sivlonos?! Regarding Sivlonos, we did not see a case, but regarding fasting, we did! Shiltei ha'Giborim said that perhaps there is no proof, for silence (not protesting) is like admission. He understood that the Halachah was already fixed like R. Gamliel, and no one protested. This is wrong. If so, why did R. Yochanan ben Nuri say that all the days that R. Gamliel was alive, we fixed the Halachah like him. Even if we did not fix it, he should have said concisely that the Halachah follows like him because the custom is to eat and not complete the fast. Rather, R. Yehoshua wanted to annul R. Gamliel's words, for R. Yehoshua thought that the Halachah is that we complete the fast, and there was not yet a custom. R. Yochanan said that all the days of R. Gamliel our custom was to rule like everything he said. We did not scrutinize (to see if we should accept his words). We do not heed you (R. Yehoshua) to annul his words, for the Halachah was fixed like him in everything, and no one protested. This is why Maharik said 'R. Yehoshua did not answer that not seeing is not a proof, for the testimony was only that he did not hear a protest, since he did not say that Chachamim agreed to him.' If so, the same applies to Sivlonos, and regarding the custom that women do not slaughter (see below).

Shiltei ha'Giborim (ibid.): Maharik means that when there was an argument, and afterwards there was a custom, the custom is like testimony that it is the custom. He does not mean that merely not seeing makes a custom. This is clear from his words and the episode he discusses. Further, if we will not say so, Zevachim 103b would refute his proof from Eruvin! Also, we can say that not seeing is a proof when they refrain at the time of the act (Sivlonos were given, and we did not say that she is Safek Mekudeshes). He brought a good proof from R. Gamliel, for Tish'ah b'Av occurred on Erev Shabbos and people did not finish the fast. This is like testimony that this is the custom. Really, his proof is not so solid. R. Yochanan said that the Halachah follows R. Gamliel, and no one protested. This is unlike (merely) not seeing. Not protesting is silence, which is like admission, unless the Gemara explains the silence! Also, R. Yochanan testified that the Halachah was fixed like R. Gamliel, and this was followed in practice. Regarding taxes, the Mordechai says in the name of Maharam that the Halachah is fixed only after there is a custom. R. Yochanan said that the Halachah follows R. Gamliel, for this is the custom. The Gemara supports this. It says that in the days of R. Gamliel, they followed R. Gamliel. Not seeing is a proof regarding who is considered a resident. There, if he was a resident, the others would know!

Shulchan Aruch (YD 1:1): Even women may slaughter.

Rema: Some say that we do not let women slaughter, for their custom is not to slaughter. This is the custom, that women do not slaughter.

Agur (1062): Even though the Poskim hold that women may slaughter l'Chatchilah, the custom in all of Chutz la'Aretz is that they do not. I never saw a custom that they slaughter.

Rebuttal (Beis Yosef DH u'Mah she'Chosav Nashim): If they wanted to slaughter and people did not let them, this could be called a proof. Not seeing it not a proof!

Shach (1): The Agur holds like Maharik, that regarding a custom v'Chi Hai Gavna (or similar cases), if something was not seen, this is a proof.

Rebuttal (Tumim 24): Surely, when it is known that the custom in some places is a certain way, or Poskim argue, not seeing is a proof, e.g. Sivlonos. Some places are concerned for Kidushin, and some are not, like the Gemara and Poskim say. If they say that they never saw anyone concerned for Sivlonos here, this is a proof. Likewise, there was an argument whether or not to complete the fast. However, no one disapproved of women slaughtering. How can one say that because he did not see it, the custom is that they do not slaughter?! We cannot make a custom based on this, like the Beis Yosef said. This requires investigation.

Sifsei Da'as: Some say that the text of the Shach is Ki Hai Gavna (without a Vov, i.e. regarding such a custom), i.e. something common (the Shach does not add similar cases).In CM (brought below) he says that when there is no contradiction, not seeing is a proof! Rather, the text has a Vov. Not seeing is a proof when there is a common custom, or a similar case in which no one contradicts.

Machatzis ha'Shekel (DH veha'Beis): The Mechaber argues with the Agur, so he did not bring his opinion. The Rema holds like the Agur here and in CM.

Kaf ha'Chayim (10): Mekom Shmuel defends the Beis Yosef. I agree, for when a Posek says 'we did not see', he refers to his country and nearby lands. He does not discuss the entire world! However, sometimes a Posek connotes that he discusses the entire world.

Rema (CM 37:22): For anything that depends on the custom of the city, two witnesses are not like 100. Rather, we follow the majority, for we do not need real testimony. Similarly, in such a case we do not say that not seeing is not a proof. Rather, it is a proof.

Darchei Moshe (7:17): Maharik says that for something that depends on the custom of the city, or does not require proper testimony, not seeing is a proof.

Shach (38): Someone challenged what I wrote in YD. He says that Maharik and the Rema discuss something that one wanted to do and refrained due to the custom. Shiltei ha'Giborim says that this is only when there is a custom, e.g. not to say certain Berachos or not to finish a fast. He erred. Maharik and the Rema hold like I say. We cannot say that Maharik says that not seeing is a proof only when one wanted to do and refrained due to the custom. He said 'we say that not seeing is not a proof only when two pairs of witnesses contradict each other, i.e. one pair saw Kidushin, and the other did not see. When there is no contradiction, e.g. if people testify and say that they never saw or heard anyone stringent about Sivlonos, surely we rely on them, like we find in Eruvin.' This implies that when there is no contradiction, not seeing is a proof, similar to witnesses who did not see Kidushin. They do not say that they saw people who refrained from Kidushin! Also, if not, 'not seeing is not a proof' would not apply. If a case occurred and they refrained, this is the ultimate proof! Rather, Maharik discusses when we do not know whether a case occurred, just we never heard that anyone was stringent. Therefore, since the custom is to give Sivlonos, and we never heard that anyone is stringent, not seeing is a proof, since no one contradicts it. The Rema (Darchei Moshe 7:7) wrote Stam in the name of Maharik that regarding customs, not seeing is a proof. He wrote so Stam in Siman 37, i.e. not only if a case occurred and people refrained. This is because when there is a custom, cases occur often, so if it were permitted (or forbidden), it is impossible that we would not have seen once that they were lenient (or stringent). Rather, surely the custom is specifically to forbid (or permit). Sivlonos are given all the time, and people always slaughter and we never heard of a woman slaughtering or learning Hilchos Shechitah in order to slaughter. In such common cases, when there is a custom, not seeing is a proof.

Gra (32): Eruvin 41a shows that in such a case not seeing is a proof.