1) REBBI ZEIRA'S DOUBT
OPINIONS: According to Abaye and Rava, if a person commits one sin twice, in two different Ha'alamos, the two acts are considered to be one even if the Ha'alamos are not identical types of Ha'alamah (for example, one was He'elem Shabbos, and one was He'elem Melachos). The Gemara says that Rebbi Zeira (or Rebbi Yirmeyah) was in doubt with regard to this Halachah. Rebbi Zeira asked whether a person is Chayav to bring a Korban when he performed half of a Shi'ur of a Melachah while he forgot that it was Shabbos, and then he did another half of a Shi'ur of Melachah when he remembered that the Melachah is forbidden. Do two half-Melachos join to make one full Melachah?
What exactly was Rebbi Zeira in doubt about?
(a) RASHI (DH Milsa d'Peshita, DH Chalukin) explains that Rebbi Zeira was in doubt about two cases. He was in doubt whether two acts that are done in two types of Ha'alamos can combine in a case in which a person did half a Shi'ur of Melachah in each Ha'alamah, and in a case in which a person did a whole Melachah in each Ha'alamah. Even though the case Rebbi Zeira asked about was a case in which the person did half a Shi'ur of Melachah, he could have asked the same question about a case in which the person did a full Shi'ur of Melachah. In that case, does the sinner need to bring two Korbanos, or does he need to bring only one Korban? (See RAN.)
(b) The RITVA, in the name of "Yesh Mefarshim," explains that Rebbi Zeira knew for certain that one is Chayav to bring two Korbanos if he does a full Shi'ur of the transgression in each Ha'alamah. That is why he did not ask what the Halachah is in that case. Instead, he asked about a case in which one does half a Shi'ur of Melachah in each Ha'alamah. In such a case, which involves two half-Melachos, it may be necessary to have a stronger division between the acts to exempt one from a Korban. Two half-Melachos naturally attract and join to each other to form one complete Melachah, and therefore a strong division may be needed to keep them separated.
2) A "YEDI'AH" THAT SEPARATES TWO "HALF-SINS"
QUESTION: Raban Gamliel states, "Ein Yedi'ah l'Chatzi Shi'ur." This means that if a person inadvertently eats half a Shi'ur of Chelev (half a k'Zayis), finds out about it, forgets and eats another half-Shi'ur, he is Chayav to bring a Korban. The RITVA explains that a Yedi'ah (the realization that one committed a sin) is not significant and does not serve to separate two acts of a sin from each other unless it follows a complete transgression. If it does not follow a complete transgression (but only a transgression with half a Shi'ur), it is insignificant.
However, the Gemara later says that Rebbi Yochanan maintains that a Yedi'ah does serve to separate between two halves of a transgression. Rebbi Yochanan says that if a person sins by eating one and a half k'Zeisim of Chelev, and then he realizes that he ate one k'Zayis of Chelev (but he does not yet realize that he ate an additional half k'Zayis), and then he forgets and eats another half k'Zayis, the Yedi'ah that he had serves to divide the first half k'Zayis from the second, and he is not Chayav to bring an additional Chatas for the additional two half k'Zeisim that he ate. That is, even though there was no full Yedi'ah of the first half k'Zayis to separate it from the second, nevertheless the Korban from the first k'Zayis separates this half k'Zayis (for which the Korban was not brought) from the next. It seems from here that it is easier to separate two half k'Zeisim from each other than to separate two k'Zeisim, which is diametrically opposite the reasoning of Raban Gamliel! Why does the Korban serve to separate one half k'Zayis from the other in this case?
(a) In Rebbi Yochanan's case, a person sinned initially by eating one and a half k'Zeisim. When he designates a Korban for the k'Zayis, even though he does not remember the other half k'Zayis that he ate, that half k'Zayis is covered by the Korban that he brings on the full k'Zayis, since it is drawn after the Zayis that accompanied it. Since the half k'Zayis was already forgiven, it cannot be combined with the later half k'Zayis.
(b) Rebbi Yochanan's ruling is consistent with Raban Gamliel's opinion. In the case of Raban Gamliel, the person discovered that he ate half a k'Zayis of Chelev. That knowledge does not qualify as a Yedi'ah, because it is not a Yedi'ah of a full sin. In Rebbi Yochanan's case, though, the Yedi'ah came after a complete sin (of one and a half k'Zeisim of Chelev), and therefore it is considered a Yedi'ah. It separates his first consumption of Chelev -- including the additional half k'Zayis -- from the second half k'Zayis that he ate. (M. KORNFELD)