Gemara

Question (Rav Papa): Nowadays, women are stringent to consider themselves Safek Zavos. They should be allowed to immerse on the seventh (clean) day (like a Zavah! The seven days of Nidah already passed.)

Answer (Abaye and Rava): They may not due to R. Shimon's teaching;

(Beraisa): "V'Achar Tithar" -- after all the (seven clean) days. Tum'ah may not interrupt in the middle;

R. Shimon expounds "v'Achar Tithar" -- after an action (Tevilah), she is Tehorah. However, Chachamim forbid doing so, lest she come to a Safek.

Kerisus 17b (Rav Yehudah citing Rav): If Shuman and Chelev were in front of him, and he does not know which he ate, he is liable (Asham Taluy). If there was one piece (Safek Chelev or Shuman) and he ate it, he is exempt.

Question: What is his reason?

Answer #1 (Rava): He learns from "v'Asesah Achas mi'Kol Mitzvos Hash-m" - he must be Shogeg about two matters (forbidden due to Safek). It is written "Mitzvas" (singular), but we read "Mitzvos."

Answer #2 (R. Zeira): When there were two pieces, it is possible to resolve the doubt (we can examine the remaining piece). If there was one piece, we cannot.

Answer #3 (Rav Nachman): Rav obligates when there are two pieces, for the Isur is Nikva. (There is definitely Isur there).

Kidushin 5b (Beraisa): If a man gave to a woman money and said 'you are Mekudeshes to me', she is Mekudeshes;

If she gave (money to him) and said 'I am Mekudeshes to you', she is not Mekudeshes.

Question (Rav Papa): In the Reisha, he gave and said. Had she said, she would not be Mekudeshes;

In the Seifa she gave and said. Had he given, she would be Mekudeshes!

Answer #1: If he gave and she said, it is as if she gave and said, and she is not Mekudeshes.

Answer #2: If he gave and she said, it is a Safek. Mid'Rabanan, we are stringent and consider her Safek Mekudeshes.

Sotah 29a: "If meat (of Korbanos) touches anything Tamei, it will not be eaten." One may not eat Vadai Tamei, but one may eat Safek Tamei.

Contradiction: "Every Tahor person may eat meat (of Korbanos)" permits one who is Vadai Tahor, but not one who is Safek Tahor.

Resolution: When an involved party has understanding, we are stringent about a Safek. When no involved party has understanding, we are lenient.

Rishonim

Rif and Rosh (Kidushin 2a and 1:1): If he gave the money and she said the words, we are concerned mid'Rabanan.

Rambam (Hilchos Ishus 3:2): If he gave the money and she said the words, she is Mekudeshes mi'Safek.

Ran (2a DH Tanu): The Rambam does not say 'mid'Rabanan'. Perhaps no Safek Kidushin requires a Get mid'Oraisa, for we follow the Chazakah (she is single). Mid'Rabanan, we are stringent about Safek Ervah.

Pri Chodosh (YD 110 Klalei Sefek-Sefeka 1, the last DH v'Im): This implies that the Rambam holds that the Safek is mid'Oraisa. The Rambam holds that every Safek is mid'Rabanan! Rather, the Ran was unsure about a Safek about how we rule. Perhaps even the Rambam requires a Get mid'Oraisa then.

Note: Some say that the Rambam agrees that the Torah forbids Safek Kares.

Rif (Shabbos 54b): If a baby died within 30 days, Chachamim were stringent about Eshes Ach, which is Chayavei Kerisos, and forbid Yibum. We are stringent about every Safek mid'Oraisa.

Ha'Emek She'alah (68:16): The Rif and Bahag say that Chachamim were stringent about the Safek, i.e. mid'Oraisa one may be lenient.

Rambam (Hilchos Isurei Bi'ah 15:29): Why isn't a Shtuki (one whose father is unknown) forbidden to every woman, lest she is Ervah to him (e.g. his sister)? The Torah says "Al Techalel Es Bitcha Lehaznosah." Chachamim explain that if one does so (lets his daughter have extramarital relations, no one will know who is the father, and), a father will marry his daughter and a brother will marry his sister. If the Torah forbids a Shtuki to every woman, lest she is Ervah to him, such incest would not result! This teaches that we do not forbid Safek Arayos, only Vadai Ervah. If you would not say so, all orphans who did not know their parents would be forbidden to marry, lest they marry Ervah.

Rambam (Hilchos Tum'as Mes 9:12): 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.

Question (Rashba Kidushin 73a DH Mamzer): Teshuvas ha'Rambam learns from Safek Mamzer that Safek mid'Oraisa l'Chumra is only mid'Rabanan. The Torah forbids only Vadai. If so, why does one bring an Asham Taluy for a Safek Kares? One opinion obligates even when there was only one piece! Even the opinion that requires two pieces argues only about Korban. We do not find that they argue about a Safek l'Chatchilah.

Answer #1 (Kesef Mishneh): An old text of the Rambam says 'a Safek about a Chiyuv Kares is Asur mid'Oraisa, for one brings an Asham Taluy for it.'

Ginas Veradim (Klal 7 DH v'Yesh Omrim): According to the Kesef Mishneh's text, if we were lenient about a Safek that did not involve Kares, it is not clear whether we may rely on this if later it pertains to Kares.

Rebuttal (and Answer #2 - Maharit 2 YD 1 DH veha'Ta'am): An errant Talmid wrote this. When there is no Chezkas Isur, the Torah does not distinguish between light and severe Isurim (Yevamos 119a). Further, the Rambam said that the Torah is lenient about Safek Ervah and Shabbos, which have Kares! We can say that Chachamim, who require two pieces, permit a Safek without Chezkas Isur. "Mitzvos" forbids only when there are two pieces. R. Eliezer expounds the way it is written (Mitzvas) to forbid even when there is one piece.

Pri Chodosh (YD 110 Beis ha'Safek DH b'Ofen): It is reasonable that if the Torah forbids a Safek with only one piece, one brings an Asham Taluy for it! However, the Halachah is that one is liable only when there are two pieces.

Chavas Da'as (YD 110 Beis ha'Safek DH Achen): We are lenient about Safek Tamei Kodesh meat, and stringent about a Safek Tamei eating Kodshim. It is not because the former is a Lav and the latter has Kares. Rather, Safek Tum'ah is Vadai Tamei in Reshus ha'Yachid and Tahor in Reshus ha'Rabim.

Chasam Sofer (EH 1:73): The Rambam and Ra'avad hold that the Torah is lenient about the Safek (of Kil'ayim lost in a garment), even though there was Vadai Isur! Even though perhaps the thread fell, the Isur was Kavu'a. This is like we say that the Isur Melachah during Bein ha'Shemashos on Motza'ei Shabbos is Kavu'a, even though perhaps Shabbos ended. This is for Lavin. The Rambam and Ra'avad argue about a Safek Kares. The Rambam is lenient about a Safek Kares if the Isur was not Kavu'a. Kuntres ha'Safek at the end of Kapos Temarim explained this. The Rambam does not learn from Safek Mamzer, which is only a Lav, rather, from "Al Techalel Es Bitcha Lehaznosah" in Hilchos Isurei Bi'ah. The Torah was lenient about a Safek Kares that is not an Isur Kavu'a. The early Meforshim missed this.

Rambam (Hilchos Isurei Bi'ah 6:14): If she immersed on the seventh clean day, her Negi'ah (what she touched) and Bi'ah later that day are Teluyim. If she did not see the rest of the day, her Negi'ah is Tahor and they are exempt for the Bi'ah. If she saw, her Negi'ah is Tamei and they are liable for the Bi'ah. Therefore, she is forbidden to her husband until evening, lest she come to Safek.

Tosfos (67b DH Aval): R. Shimon says that Chachamim forbade immersing on day seven, lest she have Bi'ah, which is a Safek.

Aruch l'Ner (Nidah 67b DH Asur): Rashi, Tosfos and the Ran explain that R. Shimon forbids Tevilah on day seven, lest she have Bi'ah that day, which is a Safek. They hold that Safek mid'Oraisa l'Chumra is mid'Oraisa, so it is not merely a stringency to forbid Bi'ah during the day. The Ra'avad (in Ba'alei ha'Nefesh) and Rambam forbid Bi'ah 'lest she come to a Safek.' They hold that the Torah permits Sefekos. The Kesef Mishneh's text of the Rambam that forbids a Safek Kares mid'Oraisa is very difficult. The Rambam said that the Torah permits even Sefekos about Shabbos and Arayos. Rather, the Rambam obligates Asham Taluy when there were two pieces, for the Isur was Nikva. The Torah is stringent about such Sefekos. However, the Rashba and Ro'oh forbid Sefekos mid'Oraisa, and explain that Chachamim forbade the Bi'ah lest she come to Safek. This requires investigation.

Note: Our text of Ba'alei ha'Nefesh forbids Tevilah.

Poskim

Shulchan Aruch (EH 27:7): If he gave the money and she said the words, and they were not discussing Kidushin, she is Mekudeshes mi'Safek.

Chelkas Mechokek (19): The Gemara says that it is a Safek, and mid'Rabanan we are concerned.

Beis Shmuel (22): The Rambam says that it is Safek Kidushin. The Rif says that mid'Rabanan we are concerned, which implies that mid'Oraisa it is not Kidushin. The Ran and Rosh say so.

Maharit (on Rif 5b): We leave something in its Chazakah only when there is a Chazakah of the matter itself and there is no Re'usa (something that weakens the Chazakah). The Chazakah of being single has a Re'usa, for she accepted Kidushin, just there is a Safek about the Kidushin. Therefore, the Torah does not leave her on the Chazakah of being permitted. When two pairs of witnesses argue about whether or not she became Mekudeshes, she may not marry, but if she did, she need not leave (Kesuvos 22b). The Torah does not permit her l'Chatchilah, rather, only b'Di'eved. The Chazakah was weakened, but there is no Chezkas Isur.

Rebuttal (Avnei Milu'im 18): We never find a distinction in the Torah for a Safek Isur between l'Chatchilah and b'Di'eved. What is the source to follow Chazakah when there is a Re'usa, e.g. a knife was found to be dented? If we do not know which of two people became Tamei in Reshus ha'Rabim, we are Metaher both of them due to Chazakah, even though there is a great Re'usa. We learn Chazakah from Tzara'as (the Kohen declares a house Tamei after he left, based on Chazakah that the Tzara'as is still the proper size, and what entered in the next week became Tamei) even if there is a Re'usa, i.e. seven days later we see that the Tzara'as decreased.