Gemara

Question: Why did Ula teach that part of a fetal sac always contains (part of) a child? A Mishnah teaches this!

(Mishnah): If part of a fetal sac left the womb before Shechitah, one may not eat it. A sign that there was a child in a woman is also a sign in an animal.

Answer: One might have thought that not every sac has a child, and we decree when part left due to when the entire sac left. Ula teaches that this is not so.

Chulin 77a (Beraisa) Suggestion: Perhaps "you will eat every animal" includes a fetal sac even if part of it left the womb!

Rejection: "You will eat it", not a fetal sac that partially left.

Question: A fetal sac always contains a child (that was not slaughtered)! Why is a verse needed to forbid it?

Answer: Indeed, the verse is only an Asmachta.

Kidushin 39a (Rav Asi): A tradition 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.

Yoma 74a (R. Yochanan): The Torah forbids Chetzi (a partial) Shi'ur of Isurim.

(Reish Lakish): The Torah permits it.

Question (against Reish Lakish - Beraisa): "Kol Chelev" forbids Chelev of a Kvi (a Safek Chayah) and Chetzi Shi'ur.

Answer: This is an Asmachta. Surely, a verse would not include a Safek!

Rejection: The Tana holds that it is a unique species. The verse forbids its Chelev.

Rishonim

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.

Rashba (ibid.): A tradition from Sinai permits Safek Orlah in Chutz la'Aretz and forbids Vadai. This implies that in Eretz Yisrael, a Safek is forbidden mid'Oraisa. Also, in Surya we would forbid if not for a tradition to permit!

Chavos Ya'ir (192:61): The Rambam can say that normally, Chachamim were stringent about Safek mid'Oraisa. They were not stringent about Safek Orlah in Chutz la'Aretz because there was a special tradition to permit it. Even though I challenged the Taz (YD 269:117) who says that Chachamim did not forbid something (like Milah on Shabbos) that the Torah explicitly permits, regarding Safek Isur it is true. How can the Rambam say that Chachamim were more stringent about a Safek Isur (they said that it is not Batel), which mid'Oraisa is permitted, than about a Vadai Isur (which is Batel)!

Ran (Kidushin 15b DH Garsinan): The verse is needed to permit any Safek at all, even if it would not suffice to permit elsewhere. We say that in Chutz la'Aretz we follow the most lenient opinion in Eretz Yisrael, even an individual against a Rabim. We are lenient even if the Safek is close to Vadai; people may supply each other (for the recipient is not sure that he gets Orlah). For other Isurim this is forbidden due to Lifnei Iver.

Tosfos (Yoma 74a DH Itztrich): We expound verses to teach about Safek Ma'aser Behemah (Bechoros 58b) and a Safek whether or not a Nega (Tzara'as) preceded the white hair, and did not ask! A Kvi is different, for it is always the same. If the Torah came to teach about it, it should teach what it is!

Question (Maharit DH v'Iy): The Rambam holds that the Torah permits Sefekos. Why did the Gemara ask why a verse forbids Chelev of a Safek?

Answer (Maharit): Tosfos answered that this Safek is different, for it is always the same. Tosfos holds like the Rambam. He did not distinguish Sefekos in which we would need to be stringent without a verse. He must agree that mid'Oraisa, all Sefekos are permitted. Tosfos asked why we need a verse to exempt a Safek male from Korbanos Re'iyah. He cannot do Semichah, lest he is a female! Really, the Torah permits the Safek. However, if one wants to be Yotzei due to Safek, it is better to be passive and not transgress mi'Safek.

Rebuttal (Pri Chodosh YD 110 Beis ha'Safek DH veha'Ran and DH Emnam): Tosfos (Chulin 22b DH Itztrich) suggested that a Kvi is really a Safek, and the verse forbids its Chelev Vadai, not just due to Safek. This shows that Tosfos holds like the Rashba. Yoma 74a is a proof for the Rambam. Reish Lakish, who permits Chetzi-Shi'ur mid'Oraisa, also permits Chelev of a Kvi mid'Oraisa. Surely, the same applies to every Safek. Do not say that R. Yochanan forbids (all) Sefekos mid'Oraisa, for he held that the Drashah to forbid Chelev kvq is mid'Oraisa. He was forced to say that kvq is a species unto itself, for a verse would not teach about a Safek that is always the same. We conclude that every Safek about the Halachah, unresolved questions, about what occurred, even when there are two sets of opposing witnesses, is mid'Rabanan, for we follow the Chazakah. When there is no Chazakah, the Torah is lenient and Chachamim are stringent. If there is a contradiction (of leniencies), the Torah is stringent. However, if one clarify, he may not enter the Safek. Anything expounded from the 13 Midos is considered mid'Oraisa, and mid'Rabanan we are stringent about a Safek about it.

Maharit (DH v'Tu): A Beraisa permits Tum'as Kohanim only for Vadai relatives, but not for a Safek. We understand this if a Safek is normally permitted, and here it is forbidden. Those who say that the Torah is stringent about all Sefekos hold that the verse teaches that one is lashed, for Tum'as Kerovim was permitted only for Vadai. Alternatively, one might have thought that the Mitzvah of Tum'as Kerovim applies even to a Safek.

Pri Chodosh (ibid., DH v'Im Tomar d'Tanya): The verse forbids when a Chazakah says that he should be Mitamei, e.g. his wife was Safek divorced.

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.

Pri Chodosh (DH v'Im Tomar l'Da'as): The Gemara brings verses to teach about Sefekos about Ma'aser Behemah (Bava Metzia 6a), a hair in a Nega, and Tum'ah in Reshus ha'Rabim. The Rambam says that all Sefekos are mid'Rabanan! We can say that these discuss when there was a Chazakah

Pri Chodosh (YD 110 Klalei Sefek-Sefeka DH v'Im Tomar v'Ha): The Gemara (Chulin 77a) expounds that one may not eat a Shilya (if part left). Rashi explains that since it always contains a child, even without a verse we may not eat it, lest the child left and was born, i.e. we must be stringent about the Safek. We answered that the verse is only an Asmachta. Really, this is a Sefek-Sefeka, for the child is born if its majority or its head left. Bava Kama 11a said that if not every fetal sac had a fetus, we would permit due to a Sefek-Sefeka (even if it had a fetus, perhaps it did not leave). Really, there are also two reasons to forbid (perhaps the head or majority left). The Gemara means that the reasons to permit override. Alternatively, the Seifa (a sign of a child in a woman is also a sign in an animal) teaches that even part of a fetal sac has a child. Alternatively, the Gemara asked a flimsy question, for it knew that the verse really teaches about when the spleen was removed.