Pesach

Ramba"m Hilchos Chametz Umatzah 2,2- "what is the Hashbasa (destoying chametz) that is said in the Torah? That he should be mevatel the chametz in his heart, and consider it like dirt and he should place in his heart that he has no chametz in his reshus at all. And that all the chametz in his reshus should be like dirt and like something he has no use for at all." Why does the Rambam repeat that you should consider all the chametz like dirt? Also how can you consider all the chametz in your reshus to be like dirt once you've already made it as though you have no chametz in your reshus? You don't have anything in your reshus to make into dirt!