In the course of a discussion about when a person will be viewed as benefiting from building materials taken from the Temple treasury, the Gemara discusses whether a person who lives in someone’s unrented house without their knowledge is obligated to pay for the benefit that he receives.
This discussion revolves around the Talmudic axiom zeh neheneh ve-zeh lo haser – where one person is benefiting while the other is not losing anything, we generally do not obligate the person deriving the benefit to pay. The full discussion of this question takes place in Bava Kamma (20-21).
In Bava Kamma we find that Rav Sehorah quotes Rav Huna in the name of Rav saying that someone who lives in someone else’s courtyard will not have to pay him, based on the passage in Yeshayahu (24:12) u’she’iyah yukat sha’ar – abandonment destroys the gate – meaning that a place that remains uninhabited becomes destroyed. Rav Ashi claimed that he had actually seen it (the she’iyah), and that it gored like an ox.
According to Rashi, she’iyah is the name of a destructive demon that wreaks havoc on uninhabited places. Rav Hai Ga’on suggests that she’iyah refers to a type of insect that destroys wood, and when a house is left unattended the insect could destroy it entirely. This approach would help explain why the destruction begins at the gate – at the door of the house which is made of wood and is the first to be affected. After the beams that support the wood are destroyed, the roof will fall in and the house will be demolished. Rav Ashi’s statement that the she’iyah was like a goring ox may refer to the noise made by the insect as it eats and digests the wood.
The Rosh argues that this reason notwithstanding, the real reason that the uninvited tenant will not be obligated to pay is because zeh neheneh ve-zeh lo haser. The Rashba, Nimukei Yosef and others explain that there is always some minor damage done to the house by its tenants, so Rav Sehorah’s explanation is important because it clarifies that we see the tenant as contributing more to the upkeep of the place than the damage that he is invariably causing to it.