And behold, at the beginning of Tractate Pesachim (2a) it is stated: "On the eve of the fourteenth [of Nisan], one checks for leaven by candlelight." And Rashi explains (s.v. "checks"): "So that he does not transgress it unwittingly, neither seeing nor finding it." And Tosafot raised an objection (s.v. "candlelight"): "And it is difficult for Rabbi Yitzchak, since it is necessary for nullification, as stated in the Gemara (6b), the one who checks must nullify, and according to the Torah, mere nullification suffices, why then did the sages require checking at all? And it appears to Rabbi Yitzchak that even though mere nullification suffices, the sages were stringent to check for leaven and to burn it so that it does not come to be eaten. And similarly it appears later (10b), where Rava asks: if a loaf is on the roof and so on, or perhaps there are times when it falls and comes to be eaten." And that is what is stated there: "Rava asks: if a loaf is on the roof, does it require a ladder to bring it down or not. Do we say we're not concerned about it, since if it doesn't come down by itself, it won't come to be eaten, or perhaps, there are times when it falls and comes to be eaten. And if you want to say there are times when it falls and comes to be eaten, like a loaf in a pit, does it require a ladder to lift it out or not, here certainly it is not a concern that it will come out by itself, or perhaps, there are times when it is removed for its own purposes and comes to be eaten." It is found that the primary obligation of checking was instituted so that people do not come to eat leaven found on Passover, not according to Rashi's explanation.