Baba Kama 22a - אשו משום חציו או משום ממונו

The Nimukei Yosef (10a b'dafei haRif) asks his famous question, if fire is like an arrow, how can we allow one to light a fire before shabbos that continues to burn on shabbos? This should be prohibited because it is as if the Jew who lit the fire before shabbos is continuously lighting it at every moment that it is burning thereby violating shabbos? He answers that the concept of "eisho mi'shum chitzo" says that since the fire is likely to spread, all the damage that is eventually caused later is as if it were completed right now at the moment that the fire was lit. Therefore it is permitted to light a fire before shabbos, because the ma'aseh of lighting is completed immediately before shabbos. The Nimukei Yosef then proves that we cannot possibly view it as if one continuously lights the fire every moment that it is burning, because if this were the case the person who lit it should not be liable for any spreading of the fire, since at that point it is no longer in his control to take back and he is an absolute o'nes. But since we view the act to be complete immediately, when it is still in his hands to abstain from lighting it, we can hold him responsible for all later damage as if it were already done at the time of lighting.

The assumption of the Nimukei Yosef in his proof is debatable. He assumes that although one made a concious decision at the time of lighting the fire to light it, if the chiyuv for eish would be that every moment it burns is as if he were lighting it, he could not be held responsible for damages caused by the fire once the fire is out of his control since it is a complete o'nes. The Steipler (21) quotes from a Ramaban in teshuva who argues and says that if a person would knowingly put himself into a situation of o'nes where they will later cause damage, even though later on it will be done b'ones, he is chayev. Even though the act of damage is only done later at a time where he is an o'nes, he is responsible for the decision that he makes now which will cause the damage later.

The Steipler cites the gemara 27b as proof to the nimukei yosef. The gemara says that if one throws keilim off the roof at a time when there are pillows underneath to cushion the fall, he then jumps down and removes the pillows, he is patur. The act of throwing didn't cause damage since at the time the pillows were there, and the removing of the pillows is only a gerama (an indirect cause) for which one is not liable. Now, if we consider the entire act of damage to be completed when the objects were thrown from the roof, as the nimukei yosef maintains, since at that time they were not set to break, it makes sense that the person cannot be liable for throwing. But if we consider one who "shoots an arrow" to be actively causing damage at every moment, the moment after the pillows are removed and the objects hit the ground it should be considered as if the thrower smashed them at that moment and be liable. From the fact that we exempt the thrower is a proof to the nimukei yosef.