1) THE SIZE OF A DATE AND ITS PIT
QUESTION: Rav Papa asks whether the size of a Koseves (date) mentioned in the Mishnah (73b) as the minimum quantity for which a person is liable for eating on Yom Kippur refers to the date with its pit or without its pit.
What is Rav Papa's question? The Mishnah clearly states, "One who eats [the amount of food equal to] a fat date, it and its pit, is liable."
(a) RASHI (DH b'Gar'inisah) explains that Rav Papa's question addresses the meaning of the words in the Mishnah. Does the Mishnah mean to say that the Shi'ur is the size of a fat date together with its pit, or does the Mishnah mean to say that the Shi'ur is the size of a fat date or the size of its pit (as both are the same size)? (In many places the letter "Vav" means "or.") That is Rav Papa's question.
(b) TOSFOS (DH Koseves) rejects Rashi's explanation because if those were the two sides of Rav Papa's question, he would have had no such a question. He surely would have assumed that the Mishnah means the size of a date together with its pit. It is not reasonable to suggest that the Mishnah would say the size of a date or the size of its pit, because it is impossible for the two to be exactly the same size. The Mishnah would have said that whichever one is the smaller of the two is the Shi'ur for which one is liable, and we would have known that certainly one is liable if he eats the larger of the two.
Therefore, Tosfos suggests a different explanation for Rav Papa's question. Rav Papa is not questioning the words "and its pit" in the Mishnah, but the word "a fat date." When the Mishnah says that one who eats the amount of food equal to "a fat date... and its pit" is liable, perhaps "it" (a fat date) already refers to a complete date together with its pit, and when the Mishnah says "and its pit" it means a second pit, effectively making the Shi'ur one Koseves and two pits (its own pit plus an extra pit). The other possibility is that when the Mishnah says "a fat date" it means the Koseves by itself, without its pit. When the Mishnah then adds "and its pit" it means a single edible date and a single pit.
(c) Tosfos cites the ARUCH who gives another explanation in the name of RABEINU NISIM GA'ON, based on the Yerushalmi (this is also the preferred explanation of the BACH in OC 612:1). The Yerushalmi explains that when the Mishnah says that the Shi'ur of Achilah on Yom Kippur is "like a date and its pit," its intention is to exclude the hollow space in the date. Dates tend to have a hollow space between the flesh of the fruit and the pit. If that space would be included in the Shi'ur, then the Shi'ur would be larger. The Mishnah teaches a stringency: one is liable for eating the smaller Shi'ur of a date and its pit without the hollow space.
In the Gemara here, Rav Papa asks whether the hollow space mentioned in the Yerushalmi is included in the Shi'ur. When the Mishnah says that the Shi'ur is the size of a date and its pit, does it mean to exclude the hollow space between the date and the pit (that is, only "the date together with its pit"), or does it mean "without its pit" -- that is, the date without being snugly attached to its pit, but with a hollow space between them?
The BA'AL HA'ITUR also explains that Rav Papa was in doubt about whether the Mishnah means to include in the Shi'ur of a Koseves the hollow space in the date or not. However, he explains the wording of Rav Papa in exactly the opposite manner. When Rav Papa asks that perhaps the Shi'ur is "a fat date together with its pit," he means with its hollow space, because those words refer to the date the way it grows naturally. When he says that perhaps it refers to "a fat date without its pit," he means that the date and the pit have been separated from each other and the Shi'ur of Achilah is determined by combining the date with its [removed] pit -- which means that the hollow space is not taken into account.
The NETZIV (in MEROMEI SADEH) suggests that this is also the intention of Rashi. (His Girsa in the end of Rashi's comment reads, "k'Kamoha u'Kegar'inasa.")
HALACHAH: None of the Poskim (with the exception of the Ba'al ha'Itur) mention either side of Rav Papa's question. They also do not mention the pit of the date. Rather, they say that the Shi'ur of Achilah on Yom Kippur is "the size of a fat Koseves, which is a little less than a k'Beitzah" (RAMBAM, Hilchos Shevisas Asor 2:1; TUR OC 612:1 (the Tur leaves out the word "a little"); SHULCHAN ARUCH OC 612:1), which is the conclusion of the Gemara (79b).
The TAZ explains that since the Gemara concludes that the Shi'ur is just less than a k'Beitzah, the size is already known and it is no longer necessary to explain whether or not the Shi'ur of a Koseves includes the pit (or the hollow space). (The reason why the Poskim mention that the Shi'ur is "the size of a Koseves" if they also say that it is the size of just less than a k'Beitzah is merely because they want to be consistent with the terminology of the Mishnah, as the LEVUSHEI SERAD points out.)