The Rambam rules (Hil. Chametz U'matza 3:8) that if one did not nullify his chametz before the sixth hour on Erev Pesach and later found chametz, he violates "bal yeiraeh" and "bal yimatzei" (chametz may not be seen nor found), since he neither destroyed nor nullified his chametz. It appears from the Rambam that one violates the prohibition of "bal yeiraeh" already on Erev Pesach from the sixth hour and on. The Ra'avad, indeed, understood this way, and argues, since it says in the prohibition, "seven days." This is a fundamental dispute, and, according to R. Akiva Eiger (Drush Vechiddush Chullin #6), it ties in to another dispute in Zevachim, where they argue according to their respective opinions.

The Mishna in Pesachim (49a) states:

If someone is going to slaughter his korban pesach or to circumcise his son or to eat a seudat eirusin (betrothal feast) in his father-in-law's house, and remembers that he still has chametz in his house, if he can go back and destroy [the chametz] and still return to his mitzvah -- he should go back and destroy [it]. If not -- he should nullify it in his mind (bitul). Clearly the Mishna is dealing before the sixth hour, since afterwards one cannot do bitul. What would the halacha be if he remembered after the sixth hour, and he does not have time to return and still perform the mitzva?

The Magen Avraham (O.C. 445:11) says that he should return immediately to destroy the chametz, since both "tashbitu" and mila are mitzvot aseh, and the aseh of chametz is more severe since he violates it each and every minute. The Be'er Hetev and Chatam Sofer quote the Even Ha'ozer who argues on the Magen Avraham, and brings a proof against him from another case mentioned in the Mishna, that of korban pesach. The mitzva of korban pesach is an aseh that carries a penalty of karet, which should take precedence over the "regular" aseh of tashbitu, and so clearly he should continue on to do the korban pesach. And since the Mishna lists all three cases together, they should all share the same rule, and also by mila he should continue on.

But what about the rationale of the Magen Avraham? We can answer for the Even Ha'ozer based on Tosfot (29b) that one who has chametz and delays in burning it, but has in mind to burn it, does not violate "bal yeiraeh," since it is a lav hanitak la'aseh (a prohibition which is rectified by an aseh), so when he burns the chametz in the end he will not be in violation. Accordingly, he certainly should do the other mitzvah first, since he will not violate the prohibition of chametz at all according to Tosfot.

The Tzlach asks on the Even Ha'ozer, how can he offer the korban pesach while he still has chametz in his house? There is a prohibition against sacrificing the korban pesach while still possessing chametz, as it says, "You shall not slaughter My feast-offering while in the possession of chametz." (Shemot 34:25) Nor would we say that the aseh of korban pesach will push aside this lav of "Lo tishchat," since this is precisely the case that the Torah is talking about. [The principle of "aseh docheh lo ta'aseh" is only where the two mitzvot are unrelated and happen to come into conflict in a certain situation.] And so, based on the Mishna's comparison, just as regarding the korban pesach he should return to destroy the chametz, so too regarding mila!

R. Akiva Eiger answers that the comparison is incorrect. Despite the fact that korban pesach and mila are mentioned together, the law depends on the reason. If destroying the chametz would supercede korban pesach because the mitzva of "tashbitu" is stronger, then we could logically say that it also supercedes mila. However, the fact that it supercedes korban pesach is because of a side issue, and should have no bearing on mila. For example, if he were unable to fulfill mila for some technical reason, such as the road were closed, obviously he would go back to destroy the chametz. Clearly, we could not then reason that he should also not continue to do korban pesach if he doesn't have this limitation! Thus, according to R. Akiva Eiger, it could be that for korban pesach he should go back, but not for mila, despite the fact that they are listed together, against the Tzlach.

Furthermore, the Gemara (63a) says that according to Reish Lakish the prohibition of "lo tishchat" exists only if the one slaughtering has chametz in the Azarah (Temple courtyard). R. Yochanan argues that one violates "lo tishchat" even if it is in his house. Thus, according to Reish Lakish, if the person were to sacrifice his korban pesach he would not violate "lo tishchat," and so he should continue on because of the severity of korban pesach. According to the Tzlach's analogy, we should then equate mila to korban pesach, and he should not return for mila either. Although R. Yochanan will then argue about korban pesach, why should we assume that he will also argue on Reish Lakish about mila? What emerges, then, is that one should continue on to perform mila even after the sixth hour, against the Magen Avraham and like the Even Ha'ozer.

The Rambam (Hil. Chametz U'matzah 1:3) introduces a new problem, because he argues on Tosfot's principle that one does not violate the prohibition of chametz so long as he has in mind to destroy it. He writes that one who acquires chametz on Pesach gets lashes. This shows that "bal yeiraeh" is not a "lav hanitak" (since one does not receive lashes for a lav hanitak), in which case one violates the prohibition of chametz even if he intends to destroy it later. If so, how could one continue on to perform the mila according to the Rambam, after all there is a lav of bal yeiraeh? Perhaps we could still say "aseh docheh lo ta'aseh." But what about third example, seudat eirusin, where there is no aseh involved; here he clearly has to go back not to violate the lo ta'aseh. So even if in first two cases we would say aseh docheh lo ta'aseh and allow him to continue on, still the Mishna is not parallel!

R. Akiva Eiger insists, though, that the principle of uniformity is an established principle. Thus, in order to maintain uniformity, we must say the opposite, that according to the Rambam after the sixth hour he always goes back to destroy the chametz, since an aseh does not push off an aseh and a lav. The Mishna, then, which allows him to nullify the chametz in his mind and continue on, must be talking only before six, like the Magen Avraham. The Ra'avad, however, who holds that there is no lav after the sixth hour until nightfall, cannot learn this way (as proven by korban pesach, where he would continue on, since it is an aseh with a penalty of karet), and must hold the reverse, that in all cases he should go on. Thus, according to both the Mishna is uniform; according to the Rambam in all three cases he returns after the sixth hour, and according to the Ra'avad he does not.

Let us examine additional sugya, Zevachim (32b). Ulla states in the name of Reish Lakish that a person who is tameh (ritually impure) and sticks his hand into the Azarah of the Temple receives lashes, because even a partial entrance is considered entering. Rav Hoshia asks from the case of a metzora (leper) whose eighth day of purification, when he has to stick his thumb into the Azarah to have blood put on it, falls on Erev Pesach. Even if he happened to be a ba'al keri who immersed (in which case he would normally not be allowed into the Azarah even after immersion), the Sages allowed him to put his thumb in so that he could become purified and offer the korban pesach, because the aseh of korban pesach, which carries a penalty of karet, takes precedence over the aseh of distancing people who are tameh. According to Ulla, though, that a partial entrance is also considered entering, a ba'al keri who immersed and put his thumb in to the Azarah would also bear a penalty of karet, so how can he become purified?!

The Gemara answers that metzora is different. Since a leniency is already made for tzaraat (which is similarly prohibited from entering the Azarah other than for the purpose of purification) it is also made for keri. Ravina (33b) answers that a partial entrance is punishable only by lashes, not karet. Thus, the aseh of korban pesach, which bears the penalty of karet, takes precedence. Tosfot asks, however, that a tameh who comes into the Azarah violates both a lav and an aseh, and an aseh alone cannot supersede a lav and an aseh! Tosfot answers that an aseh with carries karet is more significant and can even supersede a lav and aseh.

Tosfot in Chullin (78a), however, disagrees with this. Regarding the prohibition of slaughtering an animal and its offspring on the same day, the Gemara asks, how do we know that it also applies to kodashim (sacrifices). Tosfot asks on the Gemara, the prohibition is written without qualification; why should we assume that it does not apply to kodashim? He raises the possibility that perhaps we need the pasuk for an instance of korban pesach where he already slaughtered the mother in the morning and now needs the son to offer as a korban pesach, and the pasuk is teaching that it is not allowed. Tosfot rejects this possibility, though, because sacrificing the son would also violate an aseh, and so we would know that korban pesach would not override an aseh and lo ta'aseh even without a special derivation. We see a dispute between the two Tosfot whether korban pesach can supercede a "regular" lav and aseh!

Now, the Rambam (Hil. Biat Mikdash 3:18) writes that a tameh who sticks his hand into the Temple receives "makat mardut" (Rabbinical lashes). The Ra'avad argues that it would seem from the Gemara in Zevachim that he receives lashes mi'deoraita. The dispute between the two Tosfot should depend on whether a partial entrance is deoraita or not. How? Tosfot in Zevachim was led to his chiddush that a severe aseh can override both an aseh and lav in order to explain how the aseh of korban pesach could supersede the aseh and lo ta'aseh of a partial entrance. But if we hold like the Rambam that there is no Torah prohibition, then Tosfot's question doesn't begin, and there is no need for chiddush that an aseh involving karet supersedes a lav and aseh. Thus, since the dispute between the two Tosfot depends on whether there is an issur deoraita against a partial entrance, according to the Rambam it should not supersede and according to the Ra'avad it should.

Let us return. According to Rambam, a person who is on his way to a seudat eirusin after the sixth hour must go back to destroy his chametz, and so too by korban pesach and mila, since there is both a lav after six and an aseh. But according to the Ra'avad, there is only an aseh after six, and so the aseh of korban pesach takes precedence over tashbitu. Hence, based on the parallel, also in the other cases we would not require him to go back, based on Tosfot's logic that one does not violate the prohibition of chametz if he intends to destroy.

The question of a lav after the sixth hour is connected to their other dispute regarding a partial entrance as follows: According to the Ra'avad, even if there would be lav and aseh (despite his intent) on Erev Pesach, still there is karet by korban pesach, and since he holds that a partial entrance is an issur deoraita he must come to Tosfot's conclusion in Zevachim. Thus, clearly he should continue on to offer his korban pesach, and by parallel, he should continue to the others also. But how can it be by seudat eirusin, where there is no mitzvat aseh? The Ra'avad therefore concludes that must be that there is no lav after sixth. The Rambam however, holds that the lashes in Zevachim are only derabanan, and therefore he doesn't have to say Tosfot's chiddush. Since he holds that there is a violation of chametz even where he has in mind to destroy later, there would clearly be an aseh, which would supersede seudat eirusin. Furthermore, once we say that even a severe aseh does not supersede an aseh and lo ta'aseh, we can assume, as well, that there exists a lav, and korban pesach will also not supersede the prohibition of chametz after the sixth hour, maintaining the uniformity of the Mishna. Thus, the dispute about chametz after six is dependent on the question of whether the lashes in Zevachim for a partial entrance is deoraita.

This is a typical piece of Torah worthy of R. Akiva Eiger! [He concludes, though, that this is all based on the Ra'avad's understanding that according to the Rambam there is a lav after the sixth hour. However, the Maggid Mishneh explains that the Rambam also maintains that the lav is only on Pesach itself.]