The Talmud introduces the obligation of Bedikat Chametz in the first mishnah in Masekhet Pesachim. Rishonim appear to differ as to the motivation for the obligation. Rashi writes that the concern is the prohibition of “Bal Yira’eh…Bal Yimatze”, proscribing ownership of Chametz over Pesach. The Ri (Tosafot, s.v. ohr), objects, noting that the requirement of nullification (bitul) removes any concerns of biblical violations of “Bal Yira’eh…Bal Yimatze”; rather, the fear is that chametz that is found during Pesach will be eaten.

Tosafot’s objection requires some explanation, as a more fundamental question could be posed against Rashi: as Bedikat Chametz is generally understood to be a rabbinical obligation, why bring a biblical prohibition as explanation? (The Ran indeed interprets Tosafot’s objection in this manner). The Munczaczer Rebbe, the Minchat Elazar (Responsa, I, 34), interprets Tosafot as understanding that Rashi is referring not to a biblical prohibition, but a rabbinical one, yet motivated by the issue of “Bal Yira’eh…Bal Yimatze” (he notes as well that this reading of Rashi is the subject of a dispute between the Maharshal and the Maharsha). The Ri differs, then, in understanding the rabbinical prohibition to be motivated by a concern of eating. (See Resp. Kol Aryeh, I, O.C. 20, for a halakhic ramification of the two possible motivations.)

The Minchat Elazar offers another possible explanation of Rashi’s view: According to the Pri Megadim (Petichah HaKollelet to O.C.) and others, when the Rabbis modify a biblical commandment, that modification redefines the precept even on a biblical level. Thus, the requirement of bedikah creates a possibility of “Bal Yira’eh…Bal Yimatze” even if it would not have been applicable without the enactment. (Compare Responsa D’var Yehoshua, II, 58).

The Minchat Elazar quotes another possibility, from another scholar, based on the assertion of the Ran that “bitul” basically is a declaration of intent (giluy da’at) that one wishes to divest one’s self of chametz. Perhaps a case can be made, then, that one who fails to adhere to rabbinic directives in this area is thus negating any such declaration. The Minchat Elazar rejects this suggestion, though, noting that laxity in compliance with rabbinic commandments is not equated with disregard of biblical commandments.

In the understanding of the Ran, Rashi is referring to a pre-bitul situation, in which there would presumably be then a biblical concern, and thus, even after bitul, a concern remains that the bitul may have lacked full sincerity and thus not taken effect. The Pnei Yehoshua, while disputing the notion that bedikah would be a biblical obligation, suggests that Rashi is working off of an interpretation of “Bal Yematze” that connotes that chamtez “should not be found…even after searching for it”. In other words (see Orot HaPesach, 8) the concern is not motivated by the general policy regarding indeterminate situations (safek), but rather a unique aspect of the prohibition of possessing chametz.

Within the Ran’s understanding of Rashi, it would seem that the mishnah should have presented both bedikah and bitul as equal options. Rav Avraham Yitzchak HaKohen Kook (Tov Roei to Pesachim) suggests that the mishnah, in Rashi’s understanding, chooses to emphasize the active remedy, bedikah, over the passive, mental approach of bitul, in the spirit of “l’fum tza’ara agra” (in accordance with the effort is the reward).

In a separate comment, Rav Kook notes that there is a difference between fulfilling an obligation and exempting one’s self from one. Likewise, in regard to a prohibition that requires an action to be performed, that action can be fulfilled or evaded. The destruction of chametz (the step that would follow a “successful” bedikah) is the performance of the responsibility, while bitul merely removes one from the scope of the obligation. Thus, even within a context of bitul, there is room to emphasize bedikah. (Note his further comments in this regard which draw a parallel between the destruction of chametz and that of idolatry.)