Gemara

87b (Mishnah): If one witness said 'You ate Chelev' and the person denied it, he is exempt.

Inference: He is exempt only because he denied it. Had he been silent, the witness would have been believed. This shows that one witness is believed mid'Oraisa.

Question: What is the source that one witness is believed?

Answer (Beraisa): "O Hoda Elav Chataso (one brings a Chatas if he finds out that he sinned)", but not if witnesses told him (and he denies it);

If he does not deny it he is obligated - " O Hoda Elav", no matter how he found out.

Question: If two witnesses came and he did not contradict them, we do not need a verse to obligate him!

Answer: Rather, one witness came and he did not contradict him. The witness is believed.

Objection: Perhaps the witness is not believed. Rather, his silence is like admission to the witness' words!

Support (Seifa): If two witnesses say 'You ate Chelev', and he says 'I did not eat', he is exempt;

R. Meir says, he must bring a Korban.

Question: Why do Chachamim obligate a Korban in the Reisha (when there is one witness)? It is not because the witness is believed. Normally two witnesses are believed against one who contradicts them, yet Chachamim exempt in this case!

Answer: We must say that his silence is like an admission.

Kerisus 11b (Mishnah): If they told Ploni, "You ate Chelev", he brings a Chatas.

If one witness says "he ate Chelev" and one witness says "he did not eat", or if a woman says "He ate Chelev" and another woman says "He did not eat", he brings an Asham Taluy.

Question: In the Reisha, how many people told him?

Version #1 - Answer: "They" connotes two.

Question: What does Ploni say?

Answer #1: He is silent.

Inference: If one witness says "You ate Chelev", and Ploni is silent, he is exempt;

Rejection (the middle clause): If one witness says "you ate Chelev" and Ploni says "I did not eat", he is exempt.

Inference: Had he been silent, he would be liable!

Answer #2: Ploni contradicts them (yet he is liable).

This is like R. Meir, who says that (even regarding Korban) one cannot contradict two witnesses, but Chachamim exempt.

Version #2 - Answer (to Question 8): "They (told Ploni)" can refer to one (witness):

(Mishnah): A man went overseas, and they told his wife that he died. She remarried, and her husband returned. The following fines apply: she may not remain with either man...

Even though there was only one witness, it says "they". The same applies in our Mishnah.

Question: How do we know that it refers to one witness?

Answer (Seifa): If she remarried without permission of Beis Din, she may return to her first husband.

Without permission means (without the enactment of Beis Din, rather,) according to two witnesses.

Inference: In the Reisha "with permission of Beis Din" refers to one witness.

Conclusion: Also in our Mishnah, even though there was only one witness, it says "they".

Question: What does Ploni say?

If he contradicts the witness, he would be exempt!

(Seifa): If one witness says "You ate Chelev" and Ploni says "I did not eat", he is exempt.

Answer #1: He is silent.

Question: The Reisha can be inferred from the Seifa!

(Seifa): If one witness says "You ate Chelev" and Ploni says "I did not eat"; he is exempt.

Inference: Had he been silent, he would be liable!

Answer #2: Indeed, Ploni does not contradict the witness. The Mishnah means as follows:

If they (one witness) say "You ate Chelev" and Ploni is silent, he brings a Chatas;

This is only if Ploni is silent. If he contradicts the witness, he is exempt.

Rishonim

Rambam (Hilchos Shegagos 3:1): If two witnesses say that Reuven did an Aveirah for which one brings a Chatas Kavu'a (not Oleh v'Yored) but they did not warn him, and he denies it, he does not bring a Chatas.

Rambam (2): If he was silent and did not contradict the witness, even if it was a woman who said that he transgressed, he must bring a Chatas. If one witness said 'this is Chelev' and Levi was silent and later ate it b'Shogeg, he brings a Chatas. If he was warned he is lashed. This is even though the initial testimony (about the Chelev) was through one witness.

Tosfos (Kerisus 11b DH Omru): We do not need "O Hoda Elav" to exempt when a witness tells Ploni that he ate Chelev and Ploni denies it. We know this from "Lo Yakum Ed Echad..." that one witness obligates only a Shevu'ah! Rather, "O Hoda Elav" exempts when Ploni says that he does not know.

Rambam (8:3): If Ploni ate a piece and one witness says that it was Chelev and one witness says that he did not, or if a woman says that he ate Chelev and one witness says that he did not, since the Isur was Kavu'a and Ploni does not know whether or not he sinned, he bring an Asham Taluy. Similarly, if a man had Bi'ah with a married woman and one witness says that her husband died and one witness says that he did not, they bring Asham Taluy. The same applies to Safek Gerushin, for the Isur was Kavu'a. It does not apply to Safek Kidushin, for the Isur was not Kavu'a.

Question (Lechem Mishneh): Tosfos question is difficult for the Rambam! Also, the Gemara in Yevamos concluded that the witness is not believed; the Chiyuv Korban is because silence is like admission. The Gemara in Kerisus said that we can infer the Chiyuv Chatas when he is silent from the exemption when he contradicts the witness. Perhaps the verse exempts only from Chatas, but one who is silent brings an Asham Taluy!

Answer #1 (Lechem Mishneh): Rather, we must say that the verse totally exempts. We must say that when the Rambam says 'Ploni does not know', he means that he was silent. The Rambam agrees with Tosfos that if Ploni said 'I do not know' that he is exempt.

Rebuttal (Shach YD 127:16): The Rambam holds that if he says 'I do not know' he brings a Korban. (In Hilchos Nezirus 9:17 he says that if a witness says that one of two Nezirim became Tamei and they are unsure, they bring a Korban together for the one who became Tamei.)

Answer #2 (Aruch l'Ner Kerisus 11b DH she'Me'idim): The Rambam argues with Tosfos. The verse exempts one who says 'I do not know' only from Chatas, about which it says "Hoda Elav". It does not exempt from Asham, about which it says "v'Hu Lo Yoda"! It is unreasonable to bring an Asham when there is only one witness, for it is not a Safek. If the Torah believes one witness, he should bring a Chatas. If he is not believed, he is exempt!

Chak Nosan (Kerisus 11b DH Ela d'Shasik): We infer the Chiyuv Chatas when he is silent from the Reisha. When two witnesses contradict each other and he is silent, he brings an Asham. If not for the witness who said that he did not sin, he would bring a Chatas!

Question (R. Tam, brought in Rosh Gitin 5:8): In Kidushin (66a), if one witness said that Levi's wife was Mezaneh and Levi was quiet, Abaye and Rava argue about whether or not he must divorce her. Both of them agree that the witness is believed if he said that Levi ate Chelev and Levi was quiet. It is also difficult for Rava, who believes him here even when it is no longer b'Yado, but only if it once was b'Yado!

Answer (R. Tam): Here we discuss when Levi contradicts the witness or says 'I do not know'. There it discusses when he is silent. The Gemara in Yevamos (88a) proves that this is so. We cannot prove that the witness is believed, for perhaps silence is like admission. This is if Reuven says that Levi saw it or saw Raglayim l'Davar (supporting evidence), similar to the case of 'you ate Chelev'. If not, he is silent because he does not know. The Ritzva says that even when he should have known and was initially silent and later protested, if he says that his initial silence was not an admission, rather he was thinking whether or not it is true, it is not an admission. This is like Bava Metzi'a 37a.

Rosh: There are 14 distinctions about when one witness is believed about Isurim... (13). If the owner was silent when the witness said that it is Asur, this is like admission (Kidushin 66a). Rava argues only about Devar sheb'Ervah, but regarding Taharos or Chelev he agrees. Yevamos 87b says that he the silence is like admission. This is only if the witness says that the owner saw it or saw Raglayim l'Davar, similar to the case of 'you ate Chelev'. If not, he is silent because he does not know, and one witness cannot forbid if it was not b'Yado.