Gemara

20a - Question (Rav Chisda): Reuven lived in Shimon's yard without Shimon's Da'as (knowledge or consent), must he pay rent?

If the yard is not normally rented, and Reuven does not normally rent a place, Reuven didn't benefit, and Shimon didn't lose (surely, Reuven is exempt)! If the yard is normally rented, and Reuven normally rents a place, Reuven benefited and Shimon lost (surely, Reuven must pay)! Rather, he asked about a yard that is not normally rented, and Reuven normally rents a place.

Can Reuven claim 'you didn't lose due to me!' Or, can Shimon say 'you benefited (and must pay me)!'

20b - Answer #1 (Rami bar Chama - Mishnah): (If an animal ate food in Reshus ha'Rabim, it is exempt.) If it benefits, it pays the benefit.

Rebuttal (Rava): There, the damager benefits, and the owner of the food loses. Rav Chisda asked about when Reuven benefits and Shimon does not lose!

Rami bar Chama assumes that one who left food in a Reshus ha'Rabim made it Hefker (so also he does not lose).

Answer #2 (Bei R. Ami): Since Reuven caused no loss to Shimon, he is exempt!

21a (Rav Sechorah): If Levi lived in Yehudah's yard without Yehudah's Da'as, he need not pay. "Desolation breaks the gates."

(Rav Yosef): A settled house will last.

If wood or straw was stored in the house, according to Rav Sechorah, the house would have lasted anyway, so Levi must pay. According to Rav Yosef, Levi gave benefit to Yehudah, so he is exempt.

97a (Rav Daniel citing Rav): If Reuven grabbed Shimon's slave and made the slave work for him, he is exempt.

Rav holds that a slave is like land. (He always belonged to Shimon, so Shimon should own the labor!) The case is, he used the slave when the slave does not work: If Levi lived in Yehudah's yard without Yehudah's knowledge, he is exempt.

Objection: There, Yehudah gains, for the reason of Rav Sechorah or Rav Yosef.

(Rav): If Reuven seized Shimon's boat and used it, Shimon may demand the normal rental, or the depreciation.

(Shmuel): He gets only the depreciation.

Resolution #1 (Rav Papa): They do not argue. Rav discusses a boat that is normally rented. Shmuel discusses a boat not normally rented.

Resolution #2: Both discuss a boat normally rented. Rav discusses when Reuven intended to rent it, and Shmuel discusses when he intended to steal it.

Rishonim

Rif: Rav Sechorah taught that if Levi lived in Yehudah's yard without Yehudah's Da'as, he need not pay - "desolation breaks the gates." Rav Yosef said 'a settled house will last.' They argue about a case in which Yehudah was storing wood or straw in the house. This discusses a yard that is not normally rented, even if Levi normally rents a place. This is because Levi benefits and Yehudah does not lose.

Nimukei Yosef (8b DH v'Chosav): The Rosh says that (when it is normally rented, and Levi normally pays rent) Levi cannot say 'if you rented it to someone, I would have left.' Yehudah can say 'as long as people saw you there, they assumed that it was rented, and did not ask me to rent it.'

Nimukei Yosef (9a DH b'Tzivi): The Halachah follows Rav Sechorah, for Mar bar Rav Ashi holds like him. Therefore, if Yehudah used the house only for wood and straw, Levi must pay for blackening the walls. If Yehudah's house was not being used at all, Levi benefits him (by preventing its ruin) more than he harms, so he is totally exempt.

Nimukei Yosef (Reish 9b): 'He benefits and he does not lose' does not depend on pardon. It applies even if the owner does not pardon, or if orphans own it (minors cannot pardon).

Rambam (Hilchos Gezeilah 3:9): If Levi lived in Yehudah's yard without Yehudah's Da'as, if the yard is not normally rented, Levi need not pay, even if he normally rents a place, for Levi benefits and Yehudah does not lose.

Hagahos Ashri: Shimon need not pay for what he already lived there, even if it was against Reuven's will. If David stuck food in Moshe's throat, even though if Moshe would spit it out it would be repulsive and worthless, if Moshe eats it he is liable. Even though he did not cause a loss to David, he benefited from David's loss. One who lives in another's yard is exempt only if he did not reveal that the benefit pleases him. If he revealed this, he is liable. The Ri said so in his Perush on the Sugya of one who was surrounded.

Rosh (2:6): Rav Sechorah's verse is an Asmachta. Not only is one who lived in another's yard exempt. Rather, he benefited the owner!

Poskim

Shulchan Aruch (CM 363:6): If Levi lived in Yehudah's yard without Yehudah's Da'as, i.e. Yehudah told him to leave and he did not, he must pay the full rent.

Source: R. Yonah, cited in Beis Yosef DH u'Mah she'Omar Rabeinu.

Shulchan Aruch (ibid.): If he did not tell him to leave, if the yard is not normally rented, Levi need not pay (Rema - even if he expelled Yehudah from the house b'Al Korcho and lived there. Even if Yehudah normally rented it out, just at this time he was unable to, the law depends on this time), even if Levi normally rents a place, for he benefits and Yehudah does not lose.

Source (for the Rema's law): Nimukei Yosef 9a DH v'Afilu.

Gra (16): The Shulchan Aruch did not say that the reason he is exempt is because he benefits the owner by living there. This is like the Rosh, who exempts anyone who did not cause a loss. Rav Sechorah showed that he even benefits him. However, 97a connotes that he is exempt only because he protects the house! The Yam Shel Shlomo explains that the Gemara there means that a yard is very different, for Levi causes no loss and benefits Yehudah, for he protects the house. One who uses a slave causes loss and no benefit! This is difficult. Rather, the Rashba explains that a house dweller causes a small loss due to trampling, but the gain of protecting the house overrides this. On 97a, we explain why the loss in a house is less than the gain. If there is no loss, one is exempt even if the owner is displeased. The case of a boat proves this. However, why do we say there that he pays only the loss, and here he pays all the benefit?