1) THE MITZVAH TO BURN CHAMETZ
OPINIONS: Rebbi Yehudah and the Rabanan argue about the manner in which one must destroy his Chametz. The Rabanan say that one may destroy his Chametz in any manner, even by crumbling it up and throwing it into the sea or river. Rebbi Yehudah says that one must burn his Chametz.
RASHI cites the Gemara earlier (12b) which says that even Rebbi Yehudah agrees that "at the time of its destruction" ("b'She'as Bi'uro"), one may destroy his Chametz in any manner. What is "the time of its destruction," and why is that time different according to Rebbi Yehudah?
(a) TOSFOS (12b, DH Eimasai) explains that "b'She'as Bi'uro," at the time of its destruction, refers to the time prior to midday on Erev Pesach, during which people are involved in getting rid of their Chametz. "She'Lo b'She'as Bi'uro," not at the time of its destruction, refers to the time after the prohibition of Chametz takes effect, at the end of the sixth hour on Erev Pesach, as well as throughout the festival. During that time, one who finds Chametz must burn it, according to Rebbi Yehudah. Before that time, when people are involved in getting rid of their Chametz on Erev Pesach, one may dispose of his Chametz in any manner.
The logic of Tosfos is simple. Before Pesach, there is no obligation (Mitzvas Aseh) to do any particular act with one's Chametz. Rather, getting rid of one's Chametz is merely a "Hechsher Mitzvah," a preparation for a Mitzvah. One does not fulfill the actual Mitzvah of "Tashbisu" when he gets rid of Chametz before Pesach. The Mitzvah of "Tashbisu" applies only when Pesach arrives. "Tashbisu," the requirement to destroy Chametz, is a qualifying element of the prohibition of Bal Yera'eh. That is, the prohibition of Bal Yera'eh requires that one not have any Chametz in his possession on Pesach, and "Tashbisu" is what one must do with Chametz that he finds in his possession during Pesach. In effect, "Tashbisu" makes Bal Yera'eh into a "Lav ha'Nitak l'Aseh," a prohibition that can be rectified by the performance of a Mitzvas Aseh (and thus one is not punished with Malkus for transgressing the prohibition; see Tosfos 29b, end of DH Rav Ashi).
(b) RASHI (12b, DH Rava Amar) explains that "b'She'as Bi'uro" refers to after midday on Erev Pesach, the time at which the prohibition of Chametz takes effect. At that time, one may dispose of Chametz in any manner. "She'Lo b'She'as Bi'uro," the time when one must specifically burn his Chametz according to Rebbi Yehudah, refers to the sixth hour of the day, before midday, on Erev Pesach. At that time, one must destroy his Chametz by burning it.
Rashi seems to understand that the Mitzvah of "Tashbisu" applies before Pesach arrives. "Tashbisu" is the Torah's command to prepare for the festival before it arrives. The Torah commands one to get rid of his Chametz by physically altering its composition through burning, until it is totally nonexistent. When one burns his Chametz before Pesach, he fulfills the Mitzvah d'Oraisa of "Tashbisu."
After midday of Erev Pesach, however, Chametz must be destroyed in any manner possible. Once the sixth hour has passed and the prohibitions of Bal Yera'eh and Bal Yimatzei apply (see Insights to Pesachim 4:1, and Rashi 4a, DH Bein l'Rebbi Meir), the Torah no longer requires that one get rid of his Chametz in the best way possible. Rather, if there is Chametz in one's possession at that time, one must get rid of it in any way possible. According to Rashi, once Pesach has arrived, the source for the obligation to get rid of Chametz is the prohibition of Bal Yera'eh, and not "Tashbisu." (This may be Rashi's source that Bal Yera'eh applies immediately after Chatzos, as noted in Insights to Pesachim 4:1.)
However, even according to Rashi, this does not mean that one must burn his Chametz before Pesach and not sell it, give it away, eat it, or dispose of it in any other manner. Rather, it means that if one decides to destroy his Chametz rather than merely remove it from his possession, the Torah says that he must destroy it totally by burning it (see MINCHAS CHINUCH #9). Moreover, even according to Rebbi Yehudah, the Torah allows one to be Mevatel his Chametz instead of destroying it (as Rashi says on 4b, DH b'Vitul b'Alma). Rebbi Yehudah maintains that "Tashbisu" has both implications, to burn it or to be Mevatel it in one's heart (as the RAN points out on 2a, DH Ela).
(c) The ROSH, however, has a different understanding of Rashi. He says that according to Rashi's explanation of Rebbi Yehudah's opinion, Chametz can always be destroyed in any manner. There is only one hour during which there is a Mitzvah to burn Chametz -- during the sixth hour on Erev Pesach.
The KORBAN NESANEL (#8) explains that according to the Rosh's understanding of Rashi, Rebbi Yehudah's law is a special rabbinical enactment which is linked to the rabbinical enactment to destroy Chametz one hour before midday (and not to wait until midday to destroy one's Chametz). Even though the Gemara presents a Gezeirah Shavah and a Binyan Av as the source for Rebbi Yehudah's opinion, the Korban Nesanel asserts that these are only allusions which the Rabanan used as a basis for their enactment to burn Chametz during the sixth hour.
(However, the words of Rashi (DH she'Im) imply that "she'Lo b'She'as Bi'uro" is not limited to the sixth hour.)