Gemara

23a - Question (Abaye): According to R. Yochanan, who obligates for fire like for one's arrows, why is one exempt for Tamun (something concealed)?

Answer (#1 - Abaye): If one was responsible for a fire in a yard, and the wall to the yard fell, not due to the fire (so the fire spread to the next yard), he is liable for his arrows only as far as they should have gone, i.e. until the wall.

Question: If so, one should be totally exempt for the next yard!

Answer: R. Yochanan agrees that he is liable for fire like for his property. The case is, Reuven could have stopped the fire from spreading (this is not Ones);

55b (Beraisa - R. Yehoshua): If one bent another's crop in front of a fire, he is exempt b'Yedei Adam, but liable b'Yedei Shamayim:

(Rav Ashi): The case is, Shimon's fire was destined to burn Reuven's crop. Levi covered it and made it Tamun, so Shimon was exempt for it.

60a: Chachamim learn from "Kamah" that one is liable only for exposed things.

R. Yehudah obligates fire even for Tamun.

61b (Mishnah - R. Yehudah): If Reuven burned a stack, and Kelim were inside and they burned, he pays for them;

Chachamim say, he only pays for a stack of wheat or barley (whatever it was). They admit to R. Yehudah that one who burns a building pays for everything inside, for people normally put (all kinds of) things in houses.

(Rav Kahana): They argue about when one who lit in his own premises, and it spread to Shimon's property: R. Yehudah obligates him for Tamun, and Chachamim. If he lit in Shimon's premises, all obligate him for Tamun.

Objection (Rava): If so, why does the Seifa teach that say Chachamim agree about one who burns a building? It should teach that even for a stack, Chachamim exempt only when Reuven lit in his own premises. If he lit in Shimon's premises, all agree he is liable for what was Tamun!

(Rava): Rather, they argue in both cases. If one lit in his own premises and it spread, R. Yehudah obligates for Tamun, and Chachamim exempt;

If one lit in Shimon's premises, R. Yehudah obligates for anything Tamun, even a wallet. Chachamim obligate for Kelim that are sometimes put in a stack, such as tools used for threshing and with cattle.

(Beraisa - R. Yehudah): If Reuven burned a stack, and Kelim were inside and they burned, he pays for them;

Chachamim say, he pays only for a stack of wheat or barley. We view the place of the Kelim as if it were grain. This is if he made a fire in his own premises. If he lit in Shimon's premises, all agree that he pays for everything inside.

62a (Rav): I heard a law according to R. Yehudah in our Mishnah.

(Shmuel): (If we know that Levi stole from David, but not how much, we enacted that David swears how much he took, and he collects. Similarly, one whose stack was burned can swear what was inside, and collects (according to R. Yehudah).

Rishonim

The Rif rules like Chachamim, according to Rav Kahana.

Rif (10a): The one who obligates for fire like arrows obligates because it is your property, e.g. he should have fenced the fire, and did not. Regarding damage to exposed things, it is like an ox that he did not guard.

Rambam (Hilchos Nizkei Mamon 14:8): If a fire spread and burned a stack or and Kelim were inside: if they were Kelim that are sometimes put in a stack, such as tools used for threshing and with cattle, he pays for them. If they were clothing, glass Kelim, etc., he is exempt for the Kelim.

Magid Mishneh (15): The Rambam rules like R. Yochanan, who obligates for fire even like for arrows. If so, he should exempt for Tamun only when the arrows ceased (23a)! Perhaps the Rambam explains the Gemara differently. Or, he rules like the Mishnah, which did not distinguish, against that Sugya.

Gra (CM 418:33): Rashi says that the Gemara answered that R. Yochanan obligates for fire like for arrows, and like for your property. If so, why did the Gemara ask 'what do they argue about?' The Rambam and Rif explain that also liability for arrows is due to your property. One who lights in another's property is liable for Tamun. If fire is like arrows, it is as if he lit whenever the fire damaged. When is Tamun exempt? We answered 'for example, he could have fenced the fire', so it is as if he lit in his own property. The Rambam (Stam) exempts Tamun, i.e. in every case. 'For example' is not precise; always, he could have fenced it. Perhaps his text did not say 'for example.'

Rambam (9): This refers to one who made a fire in another's premises. If he lit in his own premises and it spread to another's, he is exempt for all Kelim, but he pays (for their volume) as if the stack were full of wheat or barley.

Rambam (12): If one burns another's building, he pays for everything inside, for people normally put all kinds of Kelim and items in houses.

Rosh (6:13): The Gezeras ha'Kasuv exempting Tamun applies to one who lit in his own Reshus, like the verse discusses. If he lit in another's Reshus, he is liable for everything normal to hide, e.g. threshing boards in a stack, or anything in a house, for it is normal to put anything in houses. Rav Ashi established a Beraisa like Chachamim, so the Halachah follows them. Rav said that R. Yehudah made an enactment like for one who was robbed. This is not because he holds like R. Yehudah. It is relevant to Chachamim, when he lit in another's Reshus. Rav teaches that R. Yehudah believes one even to say that he hid a wallet in a stack.

Poskim

Shulchan Aruch (CM 418:13): One is exempt for Tamun. If one burned another's stack and Kelim were inside, even boards used for threshing that it is normal to hide in a stack, he is exempt. He pays for their volume as if they were like the stack, e.g. wheat or barley. This refers to a fire lit in one's premises that spread to another's and his arrows ceased, e.g. the wall fell not due to the fire and he could have fenced it, but did not. If he lit in another's premises, or in his own premises and his arrows did not cease, e.g. the wall fell due to the fire, he is liable for Kelim that it is normal to hide in a stack, e.g. threshing tools. In a house he is liable for everything, for it is normal to put all kinds of Kelim and items in houses.

Beis Yosef (DH v'Chosvu, citing Tosfos 61b DH Ela): "When fire will go out" connotes that he lit in his own Reshus. If he lit in another's Reshus, he is liable for Kelim that it is normal to hide, for we have no source to exempt him. Reasoning exempts for Kelim that it is not normal to hide, for he need not anticipate that someone put a wallet in a stack.

Shach (6): The Shulchan Aruch connotes that even if he knew that something not normally Tamun was there, he is exempt. In 388:1, we obligate (a Moser) for anything that he knew about! There, he overtly damaged; here he merely negligent. The Yam Shel Shlomo (6:33) obligates for everything if one overtly burned, but only for what is normal if he was merely negligent.