When A mistakenly believes B owns Blackacre and enters, does he trespass?

In the context of trespass, liability typically arises from the unauthorized entry onto someone else's property. Here, A mistakenly believes that B owns Blackacre when in fact, it may belong to someone else entirely, perhaps C. Under the law of torts, particularly regarding trespass, the key factor is not only the intent but also the ownership of the land.

A’s entry onto the property, regardless of his mistaken belief about ownership, constitutes trespass because he did not have permission to enter Blackacre. The law does not recognize a subjective belief about ownership as a valid defense against trespass. Since A entered without authorization, he is indeed subject to liability to C, the true owner of Blackacre. This principle reflects the notion that ownership rights must be respected and that one cannot simply rely on a belief that may not reflect reality.

In contrast, beliefs regarding ownership do not negate the wrongdoing involved in the unauthorized entry. Factors like reasonable belief, actual ownership, and damage are secondary to the fundamental concept that trespass occurs when a person enters land without permission, independent of their beliefs or intentions. This is why the idea that A’s belief protects him from liability does not hold under tort law. Hence, A is liable to C for the trespass