1) "HATMANAH" -- INSULATING A POT
OPINIONS: The Gemara teaches three Halachos and gives the reason for each one. There are different opinions in the Rishonim how to understand these three Halachos, based on different texts of the Gemara.
1. On Shabbos one is not permitted to do Hatmanah (to insulate a pot) in material that does not add heat, because perhaps he will find that the food has cooled off and he will re-heat it ("Shema Yarti'ach"), thereby transgressing a Melachah on Shabbos.
2. During Bein ha'Shemashos one is permitted to do Hatmanah in material that does not add heat, because the pot is still very hot at that moment and one certainly will not feel the need to reheat it.
3. On Friday, before Shabbos, one is not permitted to do Hatmanah in material that adds heat, because we are concerned that he might do Hatmanah in ashes mixed with coals. We are concerned that one who does Hatmanah in such a material might stoke the coals on Shabbos. (According to Rashi's explanation, this apparently is a Gezeirah l'Gezeirah, a decree for another decree (see Insights to Shabbos 11:2). It must be that Rashi maintains that all of these edicts were included in one general decree.)
(b) RAMBAM (Hilchos Shabbos 4:2):
1. On Friday before Shabbos one is not permitted to do Hatmanah in material that adds heat, because perhaps the pot might boil over ("Shema Yarti'ach") and he will open and close the pot to keep the boiling down and to prevent the dish from overcooking. Closing an open pot that is resting on a source of heat constitutes the Melachah of cooking. (We assume that since the Hatmanah was done while it was still day, the pot probably did not yet reach a boil before the Hatmanah was done, and he did not have a chance to adjust the heat to the desired temperature.)
2. During Bein ha'Shemashos one is permitted to do Hatmanah in material that adds heat, because by that time the pot presumably has already reached a boil and the fire has been adjusted accordingly. Since there is no concern that the pot will boil or foam over on Shabbos, there is no concern that one might open and close the pot.
3. On Shabbos one is not allowed to do Hatmanah even in material that does not add heat, because of the concern that one might do Hatmanah with ashes containing an ember that is still burning and he will stoke that ember and make it burn more.
1. Although the Ra'avad favors Rashi's explanation, he gives an alternate explanation based on the text of the Gemara that the Rambam had (which was also the Rif's text of the Gemara). The Ra'avad explains that on Friday, before Shabbos, one may not do Hatmanah in a material that adds heat, because perhaps on Shabbos he will find that the food has cooled off and he will re-heat it ("Shema Yarti'ach"). Since one is showing (by doing Hatmanah) that he wants his food to be very hot, there is a concern that on Shabbos he will forget and heat it.
2. During Bein ha'Shemashos one is permitted to do Hatmanah even in material that adds heat, because usually pots that are left on the fire until Bein ha'Shemashos become so hot that they will remain hot until the night, and thus there is no concern that one will re-heat it. (We might have thought that since the person indicates his desire for the pot to be very hot by waiting until the last moment to take it off of the fire and do Hatmanah, we should be concerned that he might re-heat it on Shabbos. In truth this is not a concern, because since the pot is so hot, it will not lose any heat.)
3. On Shabbos one is not allowed to do Hatmanah even in material that does not add heat, because of the concern that he might do Hatmanah with ashes that contain an ember that is still burning and he will stoke that ember and make it burn more. (This is the same as the Rambam's view.)
2) THE "ZAV" WHO SAW A DISCHARGE DURING "BEIN HA'SHEMASHOS"
QUESTION: The Gemara discusses the possibility that Bein ha'Shemashos might be completely day, completely night, or part day and part night. The Gemara cites the Mishnah in Zavim (1:6) to explain how each of the three possibilities can effect a Chumra: If a Zav saw a discharge during Bein ha'Shemashos on two consecutive days, it is possible that he is required to (1) bring a Korban (a Chatas ha'Of, which may be brought for a Safek), which is brought only when one experiences three consecutive discharges (either three separate discharges on one day, or one long discharge that lasts for three days). It is also possible that he does not have to bring a Korban, but only has to (2) count seven clean days, which is done when a Zav experiences two consecutive discharges. A third possibility (3) is that he is not Tamei with Tum'as Zav altogether (that is, he is considered to have experienced only one discharge, or two discharges separated by one "clean" day).
It is obvious that the status of Bein ha'Shemashos does not change. Whatever the status of Bein ha'Shemashos is today (e.g. completely night), it must have that status tomorrow. If a Zav had a discharge at the same time on two consecutive nights, we must ascribe the same status to the second Bein ha'Shemashos as we ascribe to the first. How, then, could there be a possibility that he is not Tamei at all? Either both times he saw are considered day or both are considered night, in which case he saw on two consecutive days, or both are part day and part night, in which case there is a possibility that he even saw three consecutive days. How can his discharges, seen during two consecutive periods of Bein ha'Shemashos, be considered as two discharges separated by one full day?
(a) The simple understanding of RASHI (DH Safek l'Tum'ah ul'Korban) is that the doubt about Bein ha'Shemashos actually does permit us to ascribe a completely different status to one Bein ha'Shemashos than to another. Thus, if the Zav sees a discharge at any point during each Bein ha'Shemashos (even if he sees at the exact same time during each Bein ha'Shemashos), it is possible that he saw for three days, because the first Bein ha'Shemashos might have been part day (first day's discharge) and part night (second day's discharge), and the second Bein ha'Shemashos might have been completely night (third day's discharge) or part day and part night (third day's discharge). This is the simple way of understanding Rashi.
This is problematic, however, because -- as we explained above -- if we ascribe one status to the first Bein ha'Shemashos (such as completely day), then we cannot ascribe a different status to the next Bein ha'Shemashos (such as completely night). This point can be proven both from logic and from the Gemara later (35b). For this reason, the RITVA and others (see ME'IRI, RASHASH) understand Rashi differently. They explain that Rashi also maintains that we cannot ascribe a different status to the second Bein ha'Shemashos. Rashi means that the Zav does not know exactly when during each Bein ha'Shemashos he saw the discharges. Therefore, if both Bein ha'Shemashos are part day and part night, since he does not know when he saw each discharge, it could be that he saw the first one during day (towards the beginning of Bein ha'Shemashos), and the second one during night (towards the end of Bein ha'Shemashos), in which case he is Tahor because there was a day separating the two discharges. It could also be that he saw the first one during both day and night, and the second one during night, in which case he saw on three consecutive days, or that he saw both discharges during night, in which case he saw two consecutive discharges.
(b) The RASHBAM cited by TOSFOS (DH Safek l'Tum'ah ul'Korban) explains that when the Gemara says that he saw during two periods of Bein ha'Shemashos, it does not mean two consecutive periods of Bein ha'Shemashos, but rather there was a day separating each Bein ha'Shemashos (for example, he saw a discharge on Sunday evening during Bein ha'Shemashos, and then he saw another discharge on Tuesday evening at the same time during Bein ha'Shemashos). The two discharges took place at the same time each Bein ha'Shemashos. If Bein ha'Shemashos is part day and part night, then his discharges on both Sunday and Tuesday evenings may have occurred exclusively during the day or exclusively during the night, and thus there is a full day separating them and he is Tahor. However, it is also possible that the discharges occurred at the point when day becomes night (and thus he saw on both Sunday and Monday, and on Tuesday and Wednesday), and he is therefore obligated to bring a Korban. There is no possibility, though, for him to have seen two consecutive discharges.
(c) RABEINU TAM cited by Tosfos (ibid.) explains that the only way that there can be a doubt about whether the Zav's two discharges were on three consecutive days or on two non-consecutive days is if he saw at different times during each Bein ha'Shemashos, and the time at which he saw during the first Bein ha'Shemashos was earlier than the time at which he saw during the second Bein ha'Shemashos. If the Zav saw a discharge during the beginning of the first Bein ha'Shemashos, and a second discharge near the end of the second Bein ha'Shemashos, it is possible that he saw on three consecutive days: if Bein ha'Shemashos is part day and part night, then his first discharge might have occurred while the day turned to night, and it is considered as though he saw on two consecutive days. Then, during the next Bein ha'Shemashos, when he saw a discharge a little later than the time at which he saw during the first Bein ha'Shemashos, this discharge is considered to be on the third consecutive day. It could also be that the early discharge during the first Bein ha'Shemashos occurred during the day, and the later one during the second Bein ha'Shemashos occurred during the night, and he is Tahor because there is a full day separating the two discharges.
(The only difference between this explanation and Rashi's explanation, according to the Ritva's understanding, is that according to Rashi the Zav had no idea when he saw the discharge, and it is therefore possible that he saw the first discharge at an earlier time of day than the second one. According to Rabeinu Tam, the Zav knew that he saw the first discharge at an earlier time of day than the second one.)