Gemara

(Beraisa - R. Meir): If there is a Safek whether or not a liquid became Tamei, it is Tamei. If there is a Safek whether or not a liquid was Metamei something, it is Tahor.

Nidah 2a (Mishnah - Shamai): Any woman who sees that she became a Nidah is not Tamei retroactively.

Question: What is Shamai's reason?

3a - Answer: Shamai holds that a woman [usually] feels [when Dam leaves the Makor. Since she had no Hargashah before, we assume that it came now.]

Question: Is Shamai Metaher Kesamim [blood stains found without Hargashah]?!

Answer (Abaye): Shamai admits about a Kesem. Since she did not handle a bird or pass through a meat market, we cannot attribute the blood to anything except for her.

58b (Mishnah): If she has a wound that can open up and emit blood, she is Toleh on (attributes blood to) it.

R. Akiva: Chachamim enacted to be Metamei women due to Kesamim, but to be lenient.

59a - Contradiction (Beraisa): Chachamim enacted Kesamim, to be stringent.

Resolution (Ravina): This means that Chachamim enacted Kesamim to be more stringent than the Torah. The entire law of Kesamim is mid'Rabanan.

Kesuvos 9a (R. Elazar): If a Chasan claims the Kalah had no Besulim (tokens of virginity), he is believed, and she is forbidden to him.

Question: A Sefek-Sefeka (two doubts) should permit her!

Perhaps the Bi'ah was before Kidushin. And even if it was after Kidushin, perhaps she was Anus (raped)!

Answer: He discusses a girl who was Mekudeshes before three years old. (If the Bi'ah was before the Kidushin, the Besulim would have grown back.)

16a (Ravina): Most girls are virgins when they marry, and a minority are widows. [Almost] every time that a virgin marries, there is a Kol (talk);

If a girl had no Kol, this suggests that she is not from the majority.

Rishonim

Rambam (Hilchos Isurei Bi'ah 9:2): Mid'Rabanan, any woman who sees a Kesem, even if she had no Hargashah, she is Temei'ah, as if she found blood in her flesh. The Tum'ah is a Safek, for perhaps the blood is from the Makor.

Question (Kesef Mishneh on Halachah 1): We should permit due to a Sefek-Sefeka! Perhaps the blood is not from her body. And even if it is from her body, perhaps it is not from the Makor, rather, from the Aliyah (a room on top of the Makor), and it is Tahor! I answer that even so, Chachamim were stringent because it is a severe Isur. Also, since there is nothing to attribute the blood to, they were stringent to consider it Vadai from her body, so there is only one Safek. Perhaps this is the Rambam's intent.

Question (Kreisi u'Pleisi 190:1): This is all one Safek, whether or not the blood is from the Makor! Blood from the Aliyah is like blood from the nose! Rather, I wrote above (187) that she has Chezkas Taharah. We permit any Sefek-Sefeka (even if it is weak, for it is like one Safek) due to Chezkas Taharah.

Question: Why do we forbid if Kidushin was before three years? This is a Sefek-Sefeka! Perhaps the Bi'ah was before she matured. And even if it was after, perhaps she was raped!

Answer (Tosfos Kesuvos 9a DH v'Iy): This is a single Safek, whether or not it was Ones. We permit a minor who was enticed because she has no Da'as, so it is like Ones. We do not apply the Chazakah that she was permitted, for there is publicity to rape. Since we did not hear anything, Ratzon (she consented) is considered the majority. This overrides the Chazakah.

Question: We should forbid even when there is a Sefek-Sefeka. Since Ratzon is the majority, it is as if there is no Safek of Ones, so it is a single Safek (perhaps she sinned before Kidushin)!

Answer (Ri, in Tosfos): There is not a proper majority of Ratzon. It is a majority mid'Rabanan. Therefore, we permit regarding a Sefek-Sefeka, but not when there is only one Safek.

Rashi (16a DH Safek and DH Tahor): R. Meir is stringent about a Safek whether a liquid became Tamei, for it is a Torah Safek. He is lenient about a Safek whether or not a liquid was Metamei something, for he holds that this Tum'ah is only mid'Rabanan.

Poskim

Shulchan Aruch (YD 187:5): If she has a wound in that place, we are Toleh blood on the wound.

Rema: If she does not have a fixed Veses (time when she normally sees Dam Nidah), and it is a Safek whether the blood came from the Makor or from the walls [of the womb], she is Toleh on the wound due to a Sefek-Sefeka. Perhaps the blood is from the walls. And even if it is from the Makor, perhaps it is from the wound.

Sidrei Taharah (15 DH Achar): Meil Tzedakah (67) brings from Chacham Tzvi that because there is blood of the Aliyah, and also of the wound, this is like a majority of Tahor blood. This is difficult. If we do not know that the wound emits blood, it is more reasonable to attribute the blood to the Makor, for most blood is from there! The Bach and other Acharonim rule like the Rema only when she has a fixed Veses.

Shulchan Aruch (190:1): Mid'Oraisa, a woman is not Temei'ah until she feels that blood left her flesh. Chachamim decreed about a Kesem even if she did not feel.

Question: A Kesem is a Sefek-Sefeka! Perhaps the blood is not from her. And even if it was from her, perhaps it was not from the Makor! We find such a Sefek-Sefeka in 187:5.

Answer (Taz 2): Most blood from a woman without a wound is from the Makor. Therefore, this is not considered a Safek at all. Tosfos (Kesuvos 9a) says like this. The case of the Rema (187:5) is different, for there she has a wound, and perhaps the blood is from the walls.

Toras ha'Shelamim (of the Minchas Yakov, YD 190:1): Nidah 59a connotes like the Kesef Mishneh's first answer. It says that [Chachamim enacted Kesamim to be more stringent than Torah law, lest people be lenient about Nidah itself - Rashi DH Lo]. Above, the Rema connotes that the first Safek is perhaps the blood is from the walls, but not from the wound. If not, it is all one Safek, whether or not it is from the wound!

Question (Dagul me'Revavah): In 187:5, the wound is in the Makor, so if it is from the walls, it is not from the wound!

Minchas Yakov (ibid.): I say that it is a Sefek-Sefeka because the majority was weakened. Most women do not see blood from the Makor due to Bi'ah. We find like this in Kesuvos 16b. Therefore, here it is a Sefek-Sefeka, since most women feel when they see blood. This weakens the majority [that most blood from a woman without a wound is from the Makor], so it is a proper Sefek-Sefeka, if not for the Kesef Mishneh's reasons.

Kreisi u'Pleisi (2): According to the Kesef Mishneh's first answer, why is the Rema lenient? Perhaps it is because the Rema discusses when there is a weakness (the wound) in front of us. We do not know whether it emits blood. The Rashba is lenient even if it does not emit blood. The Kesef Mishneh's latter answer is correct. The Gemara asked that since Shamai holds that a woman feels the blood, they should be Metaher Kesamim totally! It answered that since she did not engage in anything else [surely it is from her]. All the more so, we [who say that she does not always feel blood] are Metamei. In the Rema's case, there is a wound to which we attribute the blood. Minchas Yakov properly refuted the Taz. We do not say that most women see from their body, because most feel the blood, like we asked against Shamai!

Sidrei Taharah (2): I wrote (183:2 DH uv'Zeh) that even if the Sefekos are not equal, it is called a Sefek-Sefeka, unlike the Shach (53:14). If so, there is no difficulty. Toras ha'Shelamim says that the Rema discusses one who sees blood due to Bi'ah. I disagree.

Mar'eh Yechezkeil (52): Why did the Gemara ask what is Shamai's reason? Perhaps they are Metaher due to Sefek-Sefeka! This cannot be. Most blood is from the Makor, like Minchas Yakov says. The Ran says that two minorities are like a majority. If so, why did the Gemara question Shamai? Perhaps they are Metaher because she saw without Hargashah, or from the walls! Really, the questioner did not consider that most see with Hargashah. The answerer was Mechadesh this in the last answer. In Nidah 2a, we discuss Taharah regarding Terumah and Kodshim, but not regarding her husband. Regarding Taharos this is like a Safek Tum'ah in Reshus ha'Yachid. It is Tamei even if there is a Sefek-Sefeka (Taharos 6:4). However, perhaps this is only for a Tum'ah mid'Oraisa. Sometimes we are lenient about Tum'ah mid'Rabanan even regarding a single Safek, e.g. Pesachim 16a!