"shall not be seen unto you": Do not see it as being yours, but void it in your heart — whence they ruled: "If one were on his way (on the fourteenth of Nissan) to slaughter his Pesach offering, or to circumcise his son, or to eat the betrothal feast in the house of his father-in-law, and he remembered that he had chametz in his house — if he can return (home) and burn it and return to his mitzvah, he should do so; if not, he should void it in his heart.
"and there shall not be seen unto you leaven": This is the difference between Beth Hillel and Beth Shammai: Beth Shammai say: Leaven (is forbidden) in (the size of) an olive, and chametz in (the size of) a date (less than an olive). Beth Hillel say: Both in (the size of) an olive.