Concerning the examples cited by the gemara to illustrate that tvius ayin is stronger than simanim:
1) R' Chiya lost a piece of meat among wine barrels (according to the girsa of Rosh and Rif) and was allowed to eat based solely on tvius ayin. Why can't we say that the unusual place where he lost it is a siman of its own and therefore the episode proves the opposite? Cf. Pesachim 8a where it says that wine cellars don't require bedikah because we don't usually bring chametz there.
2) R' Nasan lost tekheles (presumably among kala ilan) and was allowed to use it based on tvius ayin. Why is it permissible to rely on visual evidence in such a case if tekheles can be tested chemically like we learned in Menachos 43a?
Also, a more general question on the sugya. Rashba writes based on Bava Metzia 23a that even if we rely on tvius ayin, we need to make sure that it is ?????? ???? ??? and that the person's eyes are not meshaker to him. And also the Ketzos 249.2 cites an opinion that two simanim are sufficient to identify the husband's corpse and permit the wife to remarry. So how do we distinguish between reliance on a number of discrete simanim and reliance on general tvius ayin? And how do we make sure that witnesses actually rely on tvius ayin rather than recognising a specific siman while telling the beis din that they recognise the person as a whole?
Akiva, England
1)
a. I do not see why amongst the barrels would be considered an unusual place to lose something. In the time of the Gemara, wine was a frequent everyday beverage, so wine barrels were common. See Bava Metzia 23b, where we learn that if one finds barrels of wine, one may take them because we assume they possess no recognizable mark to show whom they belong to. Clearly barrels were usual. (See also Avodah Zarah 70a and 70b, which cites examples of a Nochri found "Beinei Danei." This is the same word used in our Gemara, "Beinei Dana.) The Gemara in Bava Metzia 23b discusses whether the place where one finds a lost item is itself a Siman, which the person -- who lost the item -- can use to prove that it belongs to him. The Gemara says that it is not a Siman, because we say that just like you lost an item there, so too your friend lost an item there, so there is nothing special about saying one lost an item in a particular place.
b. The Gemara in Pesachim is referring to wine cellars in one's own house, while the Gemara here is referring to wine barrels in a public area. One does not bring Chametz into one's own cellars but people might pass by barrels in the street and lose a piece of meat there.
c. It would not be sufficient for Rebbi Nasan to prove that the item lost was Techeles, because he would also have to prove that the Techeles belonged to him.
d. The Ketzos is in 259:2, not 249:2. The opinion that he cites, that two Simanim are sufficient to identify the husband's corpse, is that of Rebbi Ozer.
e. The difference between reliance on Simanim and Tevi'us Ayin is that the person who posseses Tevi'us Ayin for an object is not necessary capable of providing specific Simanim, but he knows due to the general form of the item that it belongs to him. Rashi in Shabbos 114a (DH she'Machzirin) writes that the person who claims his lost item back on the argument of Tevi'us Ayin says, "I recognize this," and it is mine, even though he does not give a specific Siman.
f. Rashi in Chulin 96a (DH bi'Tevi'us) writes that the reason why we do not return an object to someone who is not a Talmid Chacham is that we are concerned that the claimant may have a special desire to own this item and lies and says that it belongs to him even though it does not lawfully belong to him. We rely on Tevi'us Ayin only for a Talmid Chacham.
g. We see from this that, concerning a lost item, it is of more general acceptability if one provides specific Simanim, because it is possible to verify that these are accurate. Therefore, if one can give a Siman this would be preferable to testifying in Beis Din that one possesses Tevi'us Ayin. So we need not be concerned that the witnesses really recognize a Siman, but tell the Beis Din that they possess Tevi'us Ayin, because if they do recognize a specific Siman it would represent stronger testimony than claiming Tevi'us Ayin and they would tell the Beis Din about this. If they say they have Tevi'us Ayin, we may assume that they have no specific Siman.
2)
a. We see from the case of Rebbi Nasan that Tevi'us Ayin is a strong testimony, if there is no problem of "Chemdas Mamon," lying to gain money. Therefore, he was believed to say that he possessed Tevi'us Ayin, and this is mid'Oraisa. Even though it is possible to do the chemical test of Menachos 43a, this is not necessary because Tevi'us Ayin is a strong enough proof.
b. We find that Tevi'us Ayin is acceptable in other areas, too. See Shulchan Aruch, Yoreh Deah 86:1, that one may buy eggs from a Nochri if one recognizes them and possesses a Tevi'us Ayin in them that they were laid by a Kosher chicken.
With best wishes,
Dovid Bloom