The Torah's prohibition against possessing chametz during Passover seems absolute. But the rabbis of the Mekhilta identified important exceptions based on two principles: domain and control.
The first exception: chametz belonging to a Jew that is located in the domain of a non-Jew. Even though the Jewish owner technically has the ability to go and burn it, the chametz is not in his domain. He does not control the space where it sits. Therefore it does not violate the prohibition of "it shall not be seen."
The second exception works in reverse: chametz belonging to a non-Jew that happens to be in the domain of a Jew. Even though the chametz is physically present in the Jew's space, it does not belong to him. The prohibition applies to "your" chametz, and this is not his.
The third exception is the most dramatic: chametz that belongs to a Jew, in his own domain, but buried under debris. A wall has collapsed on it. He knows it is there, it is legally his, and it sits in his house — yet he cannot reach it. He cannot burn it or remove it. The Mekhilta rules that this too does not violate the prohibition.
The underlying principle is that the Torah does not punish for situations beyond a person's practical control. Ownership alone is not enough. Physical presence alone is not enough. The prohibition requires both possession and the ability to act — chametz that is yours, in your space, and within your power to destroy.