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1. Rebbi Yochanan: In three situations the Chachamim decided based on the concept of Rov, but ruled in practice as though the case was an unquestionable certainty.
2. Similarly, if a woman miscarries and an arm or leg emerge, the Chachamim rule that it is considered a birth and is definitely Tamei.
3. The Gemara discusses how to identity a piece of meat found in an area where there are nine Kosher stores and one non-Kosher store.
4. If a woman had a miscarriage while wading through a river, we assume that she gave birth to an ordinary fetus.
5. The Gemara clarifies the rule of Rebbi Yochanan (see #1).
A BIT MORE
1. For example, the Mishnah (26a) states that if a woman gives birth to a placenta in a house, the entire house is deemed Tamei. Even though a placenta is not a body itself, it is impossible that no type of fetus did not emerge as well (and the fetus causes Tum'ah). The Chachamim treat the Tum'ah of the house like a definite Tum'ah.
2. We do not say that the arm or leg came from a fetus that is not considered Tamei (known as a "closed body"), since most hands and legs come from an ordinary fetus (which is Tamei). The Chachamim treat it as though it is definitely an arm or leg of an ordinary fetus.
3. If a person bought a piece of meat from one of the stores and does not remember which one, it is considered not kosher (because it was "Kavu'a"). If he found the meat in the area and does not know from which store it came, he may assume that it is Kosher.
4. Accordingly, she offers the usual Korbanos offered after birth, even though no one saw the actual fetus (and it may have been something that does not require her to offer a Korban).
5. Rebbi Yochanan means that in three situations if Tum'ah of a woman (besides other cases that do not involve a woman), a Mishnah or Beraisa (as opposed to a Gemara) decided based on Rov but ruled in practice as though the case was an unquestionable certainty.
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