Abaye wins the argument TB Baba Metzia 22

Today’s daf TB Baba Metzia 22 continues analyzing whether there is retroactive yayush, despair of retrieving the lost object. Rava holds that there is retroactive yayush, יֵאוּשׁ שֶׁלֹּא מִדַּעַת, and Abaye doesn’t. The last challenge will prove that Abaye is correct and there’s no such thing as retroactive yayush. 

“The Gemara suggests: Come and hear a proof from that which Rabbi Yoḥanan says in the name of Rabbi Yishmael ben Yehotzadak: From where is it derived with regard to a lost item that the river swept away that it is permitted for its finder to keep it? It is derived from this verse, as it is written: “And so shall you do with his donkey; and so shall you do with his garment; and so shall you do with every lost item of your brother, which shall be lost from him, and you have found it” (Deuteronomy 22:3). The verse states that one must return that which is lost from him, the owner, but is available to be found by any person. Excluded from that obligation is that which is lost from him and is not available to be found by any person; it is ownerless property and anyone who finds it may keep it.

“And the prohibition written in the verse against keeping an item that is lost only to its owner is similar to the allowance to keep an item lost to all people that is inferred from the verse; just as in the case of the allowance, whether there is a distinguishing mark and whether there is no distinguishing mark, it is permitted for the finder to keep it, so too in the case of the prohibition, whether there is a distinguishing mark and whether there is no distinguishing mark, it is prohibited for the finder to keep it, until there is proof that the owner despaired of its recovery. The Gemara concludes: The refutation of the opinion of Rava is indeed a conclusive refutation.” (Sefaria.org translation)

Generally a halakha follows Rava; however, this is one of the six times the halakha follows Abaye. “Gemara further notes: And the halakha is in accordance with the opinion of Abaye when he has a dispute with Rava with regard to six halakhot, as represented יֵאוּשׁ שֶׁלֹּא מִדַּעַתby the mnemonic yod, ayin, lamed, kuf, gimmel, mem: Despair that is not conscious [yeush shelo mida’at], conspiring witnesses [eidim] who are disqualified retroactively, a side post [leḥi] standing alone, betrothal [kiddushin] that is not given to consummation, revealing intent with a bill of divorce [get], and an apostate [mumar] who sins rebelliously. Although the halakha generally is in accordance with the opinion of Rava in his disputes with Abaye, in these six cases the halakha is in accordance with the opinion of Abaye.” (Kiddushin 52a, Sefaria.org translation) this explanation follows Rashi’s commentary.

The Gemara never explains what the letters of the mnemonic yod, ayin, lamed, kuf, gimmel, mem mean. We shouldn’t be surprised that not everybody agrees with Rashi’s explanation. Rebbeinu Tam explains that the lamed stands for the figuring the days of the menstrual flow after birth (laydah). The Sages of Narbona explain that the lamed stands for לא אפשר ומתכוון בענין הנאה שלא מרצונו מאיסור . I wish I could can translate their explanation, but I can’t. If somebody can help me, I would be very happy.