This was adapted from something in Rav Neventzal’s new sefer “Yerushalayim B’Moadeha – Pesach” (see Page 32, Footnote 2). (I recommend it to all, along with his othet sefarim).

One step in our preparation for Pesach is to get rid of our chametz (leavened bread). We clean, scrub, vaccuum, and then throw it all away. We do this despite the fact that we also nullify the chametz in our hearts – in this nullification we say that we now relate to the chametz “כעפר הארץ” – like the dust of the earth. Even though we think of all the Chametz that had formerly been in our posession as being ownerless like the dust of the earth, we are still obligated to search after every last bit and throw away. Why is the nullification not enough?Three times a day, after we recite the the Amida prayer, we say the following words: “ונפשי כעפר לכל תהיה” – we pray and announce that such is the degree of our humility before God that we liken ourselves to be like dust, trodden upon by everyone. Even so, for most of us, if five minutes later someone would show us a a slight lack of respect, we would be angry at that person. But why should we be angry if we truly relate to ourselves to be like dust?

We see from this that even if we announce to ourselves that we are like dust, most of us do not do this with complete honesty. Therefore, chazal required us to physically check our houses for chametz, even though we have already announced that we relate to the chametz to be like dust. (And to take the parallel a step further, since in other places we liken chametz to the evil inclination, sins, etc – it is not enough for a person to just say in their heart that they have decided not to sin anymore – in cases like these, actions definitely can speak louder than words).