Mishnah: :The evening of the 14th, check for chametz by the light of the candle: The explanation of Rashi: "In order not to violate baal yiraeh and baal yimatzeh". And they asked on him, that bedikat chametz is dirabbanan and it says in our chapter and [also] you are not exempt from baal yiraeh and baal yimatzeh for unfound chametz until bitul is done as the Torah says. And even if you say that bedikah is dioritta and just it is enough, nevertheless we established that one who checks still has to do bitul and therefore even one who didn't check at all doesn't violate baal yiraeh and baal yimatzeh because they did bitul afterwards. Therefore, bedikah is not [done] for this reason and if it was it would be bedikah for nothing. Therefore, Tosafot explains that the bedikah in the Mishnah is mid’rabbanan, and even though from the Torah one can do bitul—and that would be sufficient—they required bedikah so that a person should not stumble in eating it. And this is not to say that bedikah is not mi’doraita at all, for that is certainly not so. For when the Torah writes “tashbitu se’or”, it is not a term of bitul literally, either in its simple meaning or in its translation, but rather a language of destruction—that one should remove it from his domain. And that which is stated in the Gemara that Rabbi Akiva says, etc.—rather, certainly according to everyone bedikah is mi’doraita.And when we say in the Gemara that bitul alone is sufficient, this is what we mean: that wherever one has performed bitul, the mitzvah of bedikah is no longer incumbent upon him. And so too is implied by the language, that we say “b’bitul b’alma sagi.”And even though Rashi writes that bitul is sufficient from the fact that it says tashbitu and not teva’aru, implying that tashbitu refers to nullification in the heart—this does not reveal that Rabbeinu z”l holds that bedikat chametz is mid’rabbanan. Rather, he meant that the Torah used the term tashbitu so that it would also include the concept of bitul, and the verse teaches us that if one is anus and cannot perform bedikah, the bitul that he performs before the time of the issur is sufficient for him. However, the primary verse refers to bi’ur alone. Thus you learn that the mitzvah is specifically bi’ur, but if one already did bitul beforehand, the mitzvah no longer rests upon him. Nevertheless, the Rabbis obligated him to perform bitul in any case, because bitul removes any concern. This is how one can reconcile the explanation of Tosafot according to the apparent meaning of their words, and it is a correct approach.According to this approach, one can also reconcile Rashi, for we have explained that bedikah is mi’doraita, and that is its mitzvah. The general principle is: since on a Torah level either one alone (bitul or bi’ur) is sufficient, yet one ideally should perform both, whichever comes first is mi’doraita, and the other is mid’rabbanan. Thus concludes Ritva z”l.And Rabbeinu Asher z”l concludes in the name of Tosafot that bedikah is mid’rabbanan, lest one find chametz on Yom Tov and come to eat it.And the early authorities asked: what is the reason that the Torah was more stringent regarding bi’ur chametz than other prohibitions? They answered that here it is different, because people are not naturally separate from it, and it is forbidden even in benefit, and eating it incurs karet—which is not the case with neveilah and chazir, from which people naturally keep away.And wine for a nazir, even though he is not separate from it, is not forbidden in benefit like orlah (so it seems should be the correct reading), and kil’ei hakerem, even though people are not separate from them and they are forbidden in benefit, do not carry the penalty of karet for eating.Alternatively, chametz is different because its prohibition is time-dependent: what is forbidden now was previously permitted and will again be permitted after Pesach according to the Torah. This is not the case with orlah, kil’ei hakerem, or basar b’chalav. And although wine for a nazir is also time-dependent, it is not forbidden in benefit. Therefore, since the Sages saw that the Torah was so stringent, they extended the stringency by requiring bedikah for chametz that is found, and bitul for that which is not found. Thus says the Ritva z”l in the name of his teacher, Rabbeinu Asher z”l.