A. The Rambam writes: "Chametz on Pesach, even though it is a Torah prohibition, is not included in these [prohibitions of mixtures], because the mixture is never forbidden, since after Pesach the entire mixture will be permitted, as we have explained. Therefore, it is forbidden in any amount, whether in its own kind or not in its own kind."

This is puzzling, since we hold like Rabbi Shimon, who says that chametz after Pesach is forbidden by rabbinic decree. And it can be said that the Rambam holds that the prohibition of chametz after Pesach is a new prohibition, and the prohibition of Pesach has already been lifted.

B. The Rambam also writes: "One transgresses the prohibition of 'bal yira'ah and 'bal yimmatza' [on Pesach] regarding a mixture of chametz."

Some have interpreted this to mean that one only transgresses if there is enough chametz in the mixture for a kezayit (the minimum amount for eating). Others have interpreted it to mean that one transgresses even if there is not enough chametz in the mixture for a kezayit.

C. The Rambam also writes: "The Sages prohibited eating chametz from the beginning of the sixth hour in order that one not come to transgress a Torah prohibition. And from the beginning of the sixth hour, chametz is forbidden to be eaten or enjoyed for the entire sixth hour by rabbinic decree, and the rest of the day, from the seventh hour and onward, it is forbidden by the Torah. In the fifth hour, one may not eat chametz, as a precautionary measure against the possibility of a cloudy day, when one might be mistaken between the fifth and sixth hours."

The Gra"z explains that the prohibition of eating in the fifth hour is different from the prohibition of eating in the sixth hour. In the fifth hour, it is a prohibition of the object (chametz), while in the sixth hour, they made it like a Torah prohibition of the person (i.e., one is liable for lashes). Therefore, one is only liable for lashes from the sixth hour onward.

D. The Rambam also writes: "It is a positive commandment from the Torah to destroy chametz before the time of its prohibition to be eaten, as it is said (Shemot 12:15): 'On the first day you shall remove chametz from your homes.' And from the Oral Tradition we learned that this 'first' is the fourteenth day."

The Minchat Chinuch discusses at length whether the mitzvah of "tishbittu" (destroying chametz) is a negative commandment (i.e., not to have chametz in one's possession at the time of the prohibition) or a positive commandment. He concludes that Rashi and the Rambam hold that it is a negative commandment, and Rabbi Yehuda also holds this. The requirement to destroy chametz by burning does not stem from the mitzvah of tishbittu, but rather from the analogy to noteret (leftover sacrifices). The Tosafot hold that the mitzvah of tishbittu is a positive commandment for everyone. The Minchat Chinuch lists several practical differences between these two views.

The book מועדים וזמנים (Mo'adim u'Zmanim) argues that according to the Rambam, Rashi, and the Ran, there are two parts to the mitzvah of tishbittu. One is a negative commandment, that the chametz be removed from one's possession, and the other is a positive commandment, to destroy it through an act, as implied by the language of "tishbittu." The author adds that there are also two ways to nullify chametz. One is to nullify it in one's heart, in which case the chametz remains one's own but one no longer transgresses on it, and the other is to nullify it by saying "kol chamira," in which case it becomes ownerless and leaves one's possession entirely.

The Gra"ch argues that according to Rabbi Yehuda, who holds that the mitzvah of tishbittu is to burn chametz, the mitzvah is related to the object (chametz) and is a positive commandment. According to the Sages, who hold that the mitzvah of tishbittu can be fulfilled by any means, the mitzvah is primarily that the person (i.e., the Jew) not have chametz, and therefore it is dependent on a negative commandment.

The Avnei Nezer concludes that the mitzvah of tishbittu must be performed actively, and not just by not having chametz in one's possession.

The Chofetz Chaim innovates that the mitzvah of tishbittu includes two parts. The first is to remove the chametz from one's possession, in which case one no longer transgresses on the prohibitions of 'bal yira'ah' and 'bal yimmatza.' The second is to destroy it from the world, and the method of destruction is disputed between Rabbi Yehuda and the Sages.

The Peri Yitzchak writes that one who did not destroy chametz on the first day transgresses the verse "On the first day you shall remove chametz from your homes," but he still has the mitzvah of tishbittu all seven days of Pesach.

In the novelties of Rabbi Shmuel's interpretation, he raised the view of Maimonides, that as long as the obligation of nullifying [leaven] has not taken effect, one can give it to a non-Jew and be exempt from the mitzvah. However, from the moment the obligation of nullifying takes effect and onwards, the obligation of nullification has already taken effect upon him, and he must physically eliminate the leaven from the world, and giving it to a non-Jew will not help, whether according to the sages who hold that nullification applies to everything, or according to Rabbi Yehuda who holds that nullification is by burning.

In the book of Moedim and Zemanim, he raised the view of Maimonides, that a person who has leaven in his possession after the time of prohibition, violates "not seen" and "not found" once, not like the Tosafot who hold that he transgresses every moment that the leaven is in his possession. And in the novelties of Rabbi Shmuel, he raised that one transgresses the obligation of removing it from his possession every moment, but regarding the law of nullification that relates to the object, he does not transgress as long as the law of the object is ultimately fulfilled.

The Chazon Ish concludes, that by burning every olive-sized portion of leaven, one fulfills the obligation of nullification. And the opinion of the Or Sameach is that as long as one has an olive-sized portion of leaven, his obligation is not yet complete.

The Peni Yehoshua wrote, regarding an Israelite who entrusted his leaven to a non-Jew, if he violates "not seen" depends on the dispute of the Rishonim. The opinion of Rashi and the Ramban and some of the Geonim is that he does not violate "not seen." And the opinion of Maimonides and Rabbeinu Yonah is that he does violate, and the Rash explains their reasoning, that when the non-Jew designates a place for the Israelite's leaven, it implies that according to Rashi and his contemporaries, even if the non-Jew designates a place, he does not violate. And if a non-Jew entrusts his leaven to an Israelite, if he accepts responsibility, he violates according to the opinion of Rashi, even if he designates a house for him, and according to the opinion of the Tosafot, he does not violate if he designates a house for him.

In the book of Moedim and Zemanim, it is stated that one who does not have a house is not obligated in the mitzvah of nullifying leaven, and the implication is for children by their father who are not obligated in this mitzvah.

Regarding what the Shulchan Aruch wrote about checking the leaven, the Mishnah Berurah explained: "And one also needs to nullify the leaven at that time. And from the Torah, one of them is sufficient, for when one nullifies it in his heart and forsakes it, it is no longer his and he does not transgress. And certainly when he checks it, searches for it, and burns it from the world. However, our Sages were stringent that one is not sufficient but both are necessary, because they were concerned that since nullification depends on human thoughts and intentions, perhaps one might feel comfortable having leaven for several thousands [of currency], to abandon it, and even though he may say with his mouth that it will be null and ownerless and considered as dust, nevertheless, his heart does not think so, and he does not nullify it completely, and thus he transgresses "not seen", because he did not remove it from his house. Furthermore, since one is accustomed to it all year round, if it remains in his house and possession, we decree that he might forget and come to eat it. Therefore, our Sages ordained that even though one nullifies it, it is not sufficient, but he must check to burn it from the world. Nevertheless, one also needs to nullify it, lest he does not check properly and finds leaven on Passover and transgresses upon it."

Now, it is clear that one should not rely solely on selling the leaven without checking and nullifying it, because there is still concern that he might come to eat it, and even if nullification is added to the sale, it still does not serve from this reason. However, if one checks and sells without nullification, there is room to say that it is effective, since checking nullifies the concern of him eating it, and the sale nullifies the concern of transgressing "not seen", because it is no longer his. However, ideally, one should nullify it as per the enactment of our Sages, but if one forgets, he should not trouble his heart.

Regarding what the Mishnah Berurah wrote, that the reason for checking is that perhaps nullification will not be effective when he has leaven worth thousands, one needs to examine, in the case of one who only has crumbs of leaven worth a few cents, if nullification is sufficient for him, since he does not have the concern mentioned by the Mishnah Berurah, and also he will not come to eat these crumbs, and it is clear to him that he has no attractive piece in his house, or perhaps he still needs to nullify as per the enactment of our Sages. And there is another reason he will need to check as well, because on Passover, even a minute amount of leaven is prohibited, and it is not nullified even by a thousand, and if a crumb of leaven falls into his pot and he did not nullify it before the holiday, all his food will become prohibited.