Gemara

Question: What is the source for following the majority?

Answer #1 (Mar brei d'Ravina): We learn from Korban Pesach. One may not break a bone, so we may not cut the head to check if the membrane around the brain was pierced. We must rely on the majority of animals, which are not Treifah!

Rejection: One can put a hot coal on the skull to burn through, and check!

Answer #2 (Rabah bar Rav Shilo): We learn from the Parah Adumah - "he will slaughter, he will burn" - just like it is slaughtered whole, it is burned whole. We cannot check if it was Treifah. We must rely on the majority!

Answer #3 (Rav Ashi): We can learn from Shechitah itself! The Torah permits eating what was slaughtered. Perhaps there was a hole where the knife cut! Rather, we rely on the majority of animals that are healthy.

Kidushin 39a (Rav Asi): A tradition from Moshe from Sinai forbids Orlah in Chutz La'aretz.

Question (R. Zeira - Mishnah): In Surya, Safek Orlah is permitted. (If Orlah of Chutz la'Aretz is forbidden mid'Oraisa, we must be stringent about a Safek!)

Answer (Rav Asi): The tradition permits Safek and forbids Vadai.

73a (Mishnah): Mid'Oraisa, a Shetuki (one who recognizes his mother but not his father) is Kosher. It says "Lo Yavo Mamzer (bi'Khal Hash-m)." A definite Mamzer is forbidden,but a Safek Mamzer is permitted;

A Mamzer may not marry a Vadai member of Kehal Hash-m (someone of proper lineage), but he may marry a Safek.

Rishonim

Rambam (Hilchos Tum'as Mes 9:11): If a Nefel (miscarriage) is in a pit in which weasels are common, and we do not know whether or not the Nefel is Metamei (and someone towered over it), we are lenient about the Safek.

Rambam (12): All these and similar Tum'os are mid'Rabanan. Mid'Oraisa, only one Vadai Tum'ah is Metamei. Every Safek about Tum'ah, forbidden food, Ervah and Shabbos is mid'Rabanan. Even so, a Safek about a Chiyuv Kares is Asur mid'Oraisa, for one brings an Asham Taluy for it.

Rebuttal (Ra'avad): This is a great mistake. We are stringent about a Safek mid'Oraisa, and this is a Safek mid'Oraisa! Rather, we are lenient because it is a Sefek-Sefeka, so even for a mid'Oraisa law we are lenient.

Question (Kesef Mishneh): In Sof Hilchos Kil'ayim (10:27), the Ra'avad agrees that the Torah permits Sefekos!

Answer (Maharit 2 YD 1 and Pri Chodosh YD 110, Klalei Sefek-Sefeka 1): The Ra'avad agrees that every Safek is only mid'Rabanan. The Ra'avad argues about Halachah 11. The Rambam connotes that we permit even though it is a single Safek. The Ra'avad says that if so, we would be stringent mid'Rabanan! Rather, we are lenient because it is a Sefek-Sefeka.

Question (Rashba Kidushin 73a DH Mamzer): In a Teshuvah, the Rambam learned from Safek Mamzer that 'we are stringent about a Safek mid'Oraisa' is only mid'Rabanan. The Torah forbids only Vadai. If so, how can we explain the opinion that obligates bringing an Asham Taluy for a Safek Kares, even when there were not two pieces (one of which was Vadai Asur)? Even the opinion that requires two pieces argues only about Korban. We do not find that they argue about a Safek l'Chatchilah.

Answer (Maharit 2 YD 1 DH veha'Ta'am): Perhaps Chachamim, who require two pieces, permit a Safek without Chezkas Isur. They learn from "Mitzvos" that it is forbidden only when there are two pieces. R. Eliezer expounds the way it is written (Mitzvas) to forbid even when there is one piece.

Rambam (Hilchos Yibum 2:21): Any Yevamah for whom there is a Safek mid'Rabanan whether she fell to Yibum, e.g. she gave birth to a baby born prematurely, and it died within 30 days, she should do Chalitzah mid'Rabanan. If she became Mekudeshes to a stranger before Chalitzah, she does Chalitzah and stays with her husband. If she became Mekudeshes to a Kohen, who is forbidden to a Chalutzah, he does not do Chalitzah. We do not forbid his wife to him due to a Safek mid'Rabanan.

Tashbatz (3:254): If a Yevamah gave birth to a baby, and it died within 30 days, and we are unsure whether it had a full term pregnancy, Rabanan (who argue with R. Shimon ben Gamliel) hold that mid'Oraisa, it was Vadai a viable baby (and she is exempt from Yibum and Chalitzah), for we follow the majority. Most women give birth to viable babies. However, since a minority miscarry, Chachamim are stringent for R. Shimon ben Gamliel's opinion (that it is a Safek Nefel), so she does Chalitzah. Therefore, if she married a Kohen (Chalitzah would forbid her to him), we rely on Torah law. We are not stringent on her. The Rambam says so. If it were a Safek to Rabanan, they would not let her stay with him. Tosfos asked why they were more concerned for this minority than for other minorities. He answered that it is because it is common. E.g. they were stringent about a Goses, even though most Gosesim die, or one who fell into water without end (in one or more than directions, one cannot see the shore, and we did not see him come up for longer than the amount of time that one can survive underwater). This is according to R. Shimon ben Gamliel. If it is a Safek full term pregnancy, Rabanan permit her to a Yisrael without Chalitzah. If she did Yibum, she must leave. Also the Tur (EH 164) says so.

Tur (EH 164): The Ramah says that if she did Yibum, we do not force her to leave due to Safek. This is wrong. We forbid to her to a Kohen because we rely on Rabanan. For this reasoning, she is forbidden to the Yavam! She is Eshes Ach without a Mitzvah of Yibum!

Tashbatz: This is when it is a Safek if it was a full term pregnancy. If we know that it was a full term pregnancy, all agree that the baby was viable, and she is permitted to strangers and forbidden to the Yavam. The Ramah would not permit her to stay with the Yavam. Tosfos said that most women give birth to viable babies. The Torah said that we redeem firstborns after 30 days. R. Shimon ben Gamliel says that until then, it is a Safek. Rabanan say that it is a Gezeras ha'Kasuv not to redeem until 30 days, even though the Chazakah is that the baby is viable. Similarly, we do not bring a Korban before eight days; this is not due to Safek Nefel. If we know that a baby was born after eight months and his Simanim (the hair and nails) are unfinished, the Gemara connotes that he is a Vadai Nefel. The Rashba says so. However, the Tur requires 30 days and finished Simanim to permit the Yevamah to strangers. This is like the Rosh. He did not mention a Heter to do Yibum without waiting (for the baby to die) if it is an eight month baby with unfinished Simanim. The Chazakah is that he is alive. He is not Tamei like a Nefel. He is not like like a Goses, who exempts from Yibum.

Ha'Emek She'alah (68:16): Bahag and the Rif say that if a baby died within 30 days, mid'Oraisa one may do Yibum. They argue about it is a Safek. They hold that Safek mid'Oraisa l'Chumra is only mid'Rabanan.

Ha'Emek She'alah (167:17): Rosh Hashanah 7a asked why we count the year of Bechor Ba'al Mum from the first day, since we could not eat it until eight days. If this were only a stringency mid'Rabanan, surely this is no reason to be lenient to extend the year! Rather, mid'Oraisa one may not eat within eight days. Surely, the majority of babies are viable. However, we learn from Pidyon ha'Ben (it is not until day 30) that the Torah says that we do not rely on the majority. We can say that we do not learn Mamon from Isur, and regarding inheritance we rely on the majority.

Rashba (ibid.): The Gemara (Chulin 11a) tried to prove that the Torah follows the majority from sources including Korban Pesach and Shechitah. If the Torah is lenient about a Safek, even without a majority or Chazakah these are permitted! Rather, an extra verse permits Safek Mamzer. We infer that where the Torah did not permit, a Safek is forbidden.

Maharit (ibid., DH veha'Hi): The Rambam holds that the Torah permits a Safek when it is impossible to clarify, but when one can clarify, he must. One who ate Chelev brings a Korban, for he should have been careful. We tried to prove from Pesach that we follow the majority. We rejected this, for perhaps it is only when it is impossible to clarify. Alternatively, that Gemara is like the opinion that obligates Asham Taluy even when there is only one piece. Ultimately, we brought a proof from Parah Adumah. If we did not rely on the majority, its ashes could not be Metaher against Chezkas Tum'ah. Tosfos learns from here that a majority is stronger than Chazakah.

Pri Chodosh (YD 110 Klalei Sefek-Sefeka DH Od Hevi): One opinion why Asham Taluy is only when there were two pieces is because then it is possible to clarify the Isur. Even though we hold like the opinion that it is because there was Vadai Isur, surely he agrees that one should not enter a Safek that could be avoided.

Question (Tosfos Sotah 28a DH Mah): Three verses forbid a Safek Sotah to her husband, the Bo'el and Terumah. There are no lashes for this, and in any case we must be stringent about a Safek!

Answer (Tosfos): The Torah forbids her b'Vadai, even if she did not have Bi'ah.

Merumei Sadeh (Sotah 28a DH Echad): The Rambam agrees that Safek mid'Oraisa l'Chumra when we can verify. He needs the verses for a case in which she cannot drink.

Maharit (DH u'Mah): Even if the Torah is lenient about a Safek, it must teach that we follow the majority for when there is Chezkas Isur, and to teach that we follow the majority even to be stringent.