Gemara

22a (R. Yochanan): One is liable for fire like for an arrow that he shot.

(Reish Lakish): He is liable because it is his property.

Question (Mishnah): If Reuven lit a stack of grain and there was a slave tied to it and a goat nearby, and they burned, he is exempt.

Granted, if one is liable like for arrows, he does not pay money, because he is worthy to die for killing the slave. But if one is liable for fire like for his property, why is he exempt?

Answer: The case is, he directly burned the slave.

Support #1 (for R. Yochanan - Rava): "When fire will go out" connotes by itself. "The one who burned will pay" (he is called one who burned).

Support #2 (for R. Yochanan - Rava - Beraisa): The Torah first mentioned money that damages, and concludes with one who damages. This teaches that fire is liable because it is like his arrows.

23a - Question (Abaye): According to the opinion that one is liable fire like for his arrows, why is one exempt for something concealed that is burned?

Answer (Abaye): The case is, Reuven is responsible for a fire in a yard, and the wall to the yard fell, not due to the fire (and the fire spread to the next yard). One 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 (also) liable like for his property. The case is, Reuven could have stopped the fire from spreading to the next yard (so this is not considered Ones). This is like bringing an ox into a stable and not guarding it.

Question: Since all obligate like for one's property, what do they argue about?

Answer: The one who obligates (also) like for arrows obligates the four damages.

Rishonim

Rif: R. Yochanan agrees that he is also liable like for his property. They argue about whether or not he is liable for the four damages.

Nimukei Yosef (DH Gemara): We asked about a fire that spread by itself. Is it considered like arrows that he shot without intent, through negligence? The opinions also argue about all cases in which one is liable for his animal's damage and exempt for his own. The Halachah follows R. Yochanan.

Question: If fire is like one's arrows, why may one light candles or a bonfire shortly before Shabbos, and they burn and increase on Shabbos?. It is as if he burns on Shabbos! Regarding damage, he did not intend to burn another's stack (but it is considered his action). All the more so, we should say so when he intends that it burn during Shabbos!

Answer (Nimukei Yosef): He is liable like for one who shot an arrow. Once the arrow leaves his hand, everything was finished. We do not consider what happens later to be his action. If we did, we would exempt him, for he is Anus. He cannot retract the arrow. If he died before the stack burned, surely there is Acharayos on his property to pay for it. It is like his action from when he was negligent, not like his action at the time it damages. Similarly, if one lights before Shabbos, his act finishes before Shabbos.

Nimukei Yosef (DH Tamun...): Abaye asked, if fire is like his arrows, why did the Torah exempt what is Tamun (concealed)? Surely, he would liable if he overtly burned it. The Torah obligates for Ones like for willing damage. All the more so he is liable for Tamun! Also, arrows always damage Tamun, and one is liable for them! This is not difficult if fire is like one's property. The Torah was lenient, just like it was lenient about Kelim in a pit and Shen and Regel in Reshus ha'Rabim. We answer that R. Yochanan agrees that when he is exempt for his arrows, e.g. the fire should not have passed a wall, and the wall fell, he is obligated due to his money. In this case, is exempt for Tamun. He is liable for what is exposed only if he could have stopped the fire. One is exempt for his ox when he could not guard it. We do not say that since he was initially negligent, he is liable even though the end was Ones. We say so only when he was obligated to guard the matter. This is why Levi is exempt if an ox that can see fell into Levi's pit during the day (54b). Here, he had no responsibility to guard the other Chatzer. Presumably, anything without life that is moved and damages is like one's arrows if it was destined to damage where it damaged. Such things are Toldos of fire (3b). Even though the opinions argue also about a coal that does not belong to the one who lit, we asked what the primary argument was about (it was not about this).

Nimukei Yosef (DH Ika): If one intended to throw two and it went four, he pays only Nezek. Why is fire more stringent?

Answer (Nimukei Yosef): There, he estimated how hard to throw it. He is almost Anus that it went four. Here he is negligent for not guarding his coal.

Rambam (Hilchos Nizkei Mamon 14:15): If a fire spread and damaged and wounded a man, the one who lit it pays Nezek, pain, medical expenses, Sheves (compensation for total inability to work while sick due to the injury) and Boshes (embarrassment), like one who damages through his body. Even though fire is one's property, it is like one who damaged with his arrows.

Magid Mishneh: Why didn't the Rambam explain when he is liable for Tamun? Perhaps he explains the Gemara differently. Or, he rules like the Mishnayos and Beraisos that exempt Tamun in every case, unlike our Sugya.

Lechem Mishneh: Perhaps the Rambam obligates for Boshes only when he intended to damage, like he taught in Hilchos Chovel (1:10).

Rosh (2:8): The Halachah follows R. Yochanan against Reish Lakish. Also, Rava brought a verse and a Beraisa to support R. Yochanan.

Poskim

Shulchan Aruch (CM 418:17): If a fire spread and damaged and wounded a man, the one who lit it pays Nezek, pain, medical expenses, Sheves and Boshes, like one who damages through his body. Even though fire is one's property, it is like one who damaged with his arrows.

Bach (11): Rashi (23a DH Lechaivo) exempts from paying for Boshes, for he did not intend. The Rambam obligates for Boshes. He explains that the Gemara said that R. Yochanan obligates also the other four damages.

SMA (23): The Rambam and Shulchan Aruch hold that since the fire spread through a common wind, it is as if he intended to embarrass. Even though they exempt one who fell from the roof in a common wind from Boshes, we must say that falling is a bigger Ones.

Prishah (11): The man made the fire, and it spread. This is considered with intent more than falling off a roof. The Tur obligates the four damages. It seems that he means the four other than Nezek, like in Siman 421. Also, if he excluded Boshes, he should have given the reason.

Gra (37): The Yerushalmi explicitly obligates for Boshes.

Shulchan Aruch (ibid.): This is if at the time he lit the fire, it was destined to reach the place it damaged. If not, e.g. there was a wall in between and it fell not due to the fire, even if he could have stopped the fire, his arrows ceased. He is liable for it only like for his property. He pays Nezek, but not the other four damages.