Gemara

95a (Rav): Meat that was not (constantly) watched is forbidden.

Question (Mishnah): If pieces of meat were found outside Yerushalayim, they are permitted.

Answer: Rav explains that this does not permit eating them;

(Rav): The pieces are not Tamei like Neveilah (but one may not eat them);

(Levi): One may eat them.

Question: According to Rav, how is one allowed to eat meat?

Answer #1: It was watched from Shechitah until it was eaten.

Answer #2: It is wrapped and sealed, or there is a special Siman on it.

Rabah bar Rav Huna would cut his meat on three corners.

95b - Version #1 - Question (Rav Huna): If meat on a string was not watched, what is the law?

Answer (Rav): Obviously it is a Siman (that it is the same meat. It is permitted.)

Version #2 (Rav Huna): Meat on a string is a Siman. (end of Version #2)

R. Chiya bar Avin lost the end of the intestines. R. Chanina Chuza'ah lost a back of meat. They did not have Simanim, but they recognized them. Rav Huna and Rav Nachman, respectively, permitted them.

(Rava): I used to think that a Siman is better than Tevi'as Ayin (recognition), for we return a lost object (to anyone) based on a Siman, but only a Chacham receives through Tevi'as Ayin.

Retraction (Rava): The above teachings show that Tevi'as Ayin is just as good.

Support #1 - Question: How is a blind man (or any man, at night in the dark) permitted to cohabit with his wife? (Perhaps it is a different woman!)

Answer: Surely, we rely on recognition of her voice.

Likewise, we may rely on visual recognition!

Avodah Zarah 31a: Chachamim retracted to permit closed barrels (of wine of Kusim), for Kusim guard them (for they intend to sell to them to Yisraelim).

Question (Beraisa): If one sends a barrel of wine with a Kusi, it is permitted only if he recognizes the seal.

Answer (R. Yirmeyah): He guards when everyone is making wine, for then many people are around, and he fears lest people see a Nochri touch it.

Rishonim

Rif (33b): The Halachah follows Rav.

(Ran (DH Karkesha): The episodes of the intestines and back of meat that were permitted through Tevi'as Ayin are like Rav. This does not prove that the Halachah follows him, for the cases were brought in front of Talmidim of Rav. Surely they ruled like their Rebbi.

Note: Rav Nachman was also a Talmid of Shmuel.

Ran (ibid.): Sefer ha'Terumah rules like Levi. However, he permits only in a city in which the Shochtim are Yisrael. We are not concerned for ravens from the rest of the world. Alternatively, we permit even where the Shochtim are Nochrim, if he left meat in his house and found it in the same place. In such a city, if he left meat in one place and found it elsewhere, we are concerned lest it was switched, for clearly it was taken. In the episodes in which the meat was returned through Tevi'as Ayin, it was not found where it was left. Also Levi requires Tevi'as Ayin or a Siman in such cases.

Rambam (Hilchos Ma'achalos Asuros 8:12): Even one who bought meat and put it in his house and it was not watched, it is forbidden, unless he had a Siman in it or recognizes that this is surely it, or it was wrapped and sealed.

Magid Mishneh: This is like Rav. Some say that Rav is stringent only when the meat was in a place where birds or Sheratzim could switch it, but not if he hung it on a nail or similar cases. The Ramban and Rashba say so. Perhaps the Rambam agrees. In Halachah 13, he forbids when a hanging Kli fell. It seems that if it was still hanging, it is permitted even without a Siman or Tevi'as Ayin. The Ramban says that anyone is believed through Tevi'as Ayin, not only a Chacham. The Rambam agrees. However, he must be very careful that he truly recognizes it. The Rashba and other Acharonim rule like Levi. They permit only if he left it in his house, even if most of the Shochtim are Nochrim, or even if he left it in the market, if most of the Shochtim are Yisrael. If he did not find it where he left it, letter of the law one must be concerned, since he has no Siman or Tevi'as Ayin. This is even if he left 10 pieces and found nine. The Rambam's opinion is primary.

Rambam (13): If one hung a Kli full of pieces of meat, and the Kli broke and the pieces fell and he has no Siman or Tevi'as Ayin, they are forbidden. Perhaps a Chayah or Sheretz took the meat, and this is different meat

Rivash (73): A Nochri asked a Yisrael to slaughter a calf, and he sent pieces to his Yisrael friends through a Nochri. Even Rav permits what is found in a Nochri's hand when most of the Shochtim are Yisrael. Therefore, they are permitted only if no other calf was slaughtered in the city that day.

Beis Yosef (YD 63 DH Nishal): This is unlike the Rambam.

Rosh (7:22): The Rashbam said that we rule unlike Rav when the meat was found in the same place it was left. We are not concerned lest ravens switched it and put the new meat in the same place. When a raven took meat and returned it, we are concerned lest it switched it.

Rashba (1:107): Reuven bought meat in the market and sent it with a Nochri to his house. There was no seal. He told him 'I am going away.' He does not recognize the meat. Letter of the law, it is permitted. This is obvious if most of the Shochtim are Yisrael, for this is meat found in a Nochri's hand. Even if the majority are Nochrim, perhaps we can permit b'Di'eved if the Nochri would need to go in a different direction to the Nochri meat market in order to switch it. Even though Reuven said 'I am going away', the Nochri fears lest other Yisre'elim see him, and he will appear like a thief. In Avodah Zarah, we permit closed barrels, i.e. when everyone is making wine. The Kusi fears lest people see a Nochri touch it, and he will lose. Saying 'I am going away' forbids only if he went to do something that cannot be done here. If not, the Nochri fears lest the Yisrael remember his wine and return to do it here.

Tosfos (Bava Metzi'a 23b DH Machruzos): Fish, or meat, on a string is not a Siman regarding an Aveidah (lost object). It is a Siman regarding meat that was not watched. There we are more lenient, for the Halachah does not follow Rav.

Mordechai (668): Ploni sent a slaughtered chicken with a Nochri, and said that he recognizes it. I permitted, like R. Tam, who says that the Halachah does not follow Rav.

Question (Beis Yosef DH Kasav ha'Mordechai): Even Rav permits in this case, since he recognizes it. This is better than Simanim!

Taz (63:3): The Maharshal explained that Plonis Tevi'as Ayin did not help, for he did not know who slaughtered it. The Mordechai said that the Safek was whether it was switched, not who slaughtered it! We must say that the Shochet remembers that he slaughtered a chicken for this Nochri, but did not recognize it. Even Rav would not be concerned lest the Nochri switched chickens in order to cause Ploni to transgress, since the Nochri would not benefit from this. I answer that the Mordechai holds that Rav holds that Simanim and Tevi'as Ayin do not help when there are grounds for suspicion. Here, perhaps the Nochri kept the chicken and gave an inferior chicken in its place. Levi holds that Simanim or Tevi'as Ayin helps in such cases.

Poskim

Shulchan Aruch (YD 63:1): Even one who bought meat and put it in his house and it was left unwatched, it is forbidden, unless he had a Siman in it or recognized that this is surely it, or it was wrapped and sealed

Beis Yosef (DH ul'Inyan): Since the Rif and Rambam agree, we forbid meat that was not watched if one has no Siman or Tevi'as Ayin, unless it was hung on a nail so that a Sheretz could not take it and put back (other meat).

Beis Yosef (DH v'Al): The Ramban, and Hagahos Ashri (22) citing Or Zaru'a, say that anyone is believed to say that he recognizes his meat. Regarding an Aveidah, only Chachamim (who bend the truth only in three special cases) are believed. Here, we are not concerned lest he (lie, and) feed someone else Isur. The Rashba says that R. Chananel holds that only a Chacham is believed. This is difficult, for the Gemara connotes that anyone is believed. For Isurim, we do not suspect lest he lies. However, he must be very careful that he really recognizes it. Sometimes one thinks he recognizes it, but really he does not.

Taz (1): It seems that here anyone is believed because it is a stringency mid'Rabanan.

R. Akiva Eiger: Nekudas ha'Kesef, Pri Chodosh and Tevu'os Shor say that we believe even an ignoramus even for an Isur mid'Oraisa, e.g. in a market of mostly Nochri Shochtim. Tevu'os Shor says that women are believed only for an Isur mid'Rabanan, and minors are not believed at all.

Shulchan Aruch (2): If he hung meat on a nail or similar matter that a Sheretz could not take it and return it, it is permitted. Some permit meat that was not watched if he found it where he left it.

Beis Yosef (DH v'Chasav Od): If one left meat in the market among Nochrim, Sefer ha'Terumah forbids if he has no Siman or Tevi'as Ayin. The same applies to one who sends meat with a Nochri. Rabah bar Rav Huna used to cut his meat on three corners and send it with a Nochri. Rav similarly forbids (meat sent with a Nochri with) one seal (Avodah Zarah 39a). The Rashba says that this is when most of the Shochtim are Nochrim. If the majority are Yisrael, even if one left meat among Nochrim, if he found it where he left it, we are not concerned at all lest a raven switched it.

Taz (2): If one left 10 pieces of Chametz and found nine, some say that we are concerned that they are different (and he must check the house again) only if the 10 were tied together (OC 439:3). Here is different. Chachamim decided to decree whenever we see any change in the meat, to strengthen their enactment.

Rebuttal (Shach 8 and Nekudas ha'Kesef): Here also, we forbid only if they are tied.

Rema: The custom is to be lenient like the latter opinion, even if it is in a Nochri's hand in a place where all the sellers are Yisraelim who sell Kosher meat.

Taz (3): The custom is to be lenient only if he found it where he left it.

Shach (8): If pieces of meat were found on the road where mostly Yisre'elim pass, the Rashba permits. We are not concerned for ravens as long as we do not see them take, and all the more so for meat found in a market in which most Shochtim are Yisre'elim. I say that the Rosh in the name of the Rashbam agrees. The Rosh explicitly says so in Bava Metzi'a. The Darchei Moshe, Maharshal and Drishah say that the Rosh holds like Sefer ha'Terumah, unlike the Bach.

Shach (9): The Levush says that if he is unsure whether it is in the same place, we permit even if most of the Shochtim are Nochrim. The Rosh and Poskim connote unlike this.

Gra (5): The Rema permits what is found in a Nochri's hand, and all the more so in the market.

Shach (10): If they announced that even one Tereifah was sold, one may not buy from a Nochri (who bought there). This is like a Kavu'a Safek, and all are forbidden.

Shach (11): It suffices if most of the sellers are Yisrael.