1) THE HALACHAH OF "GIREI DILEI"
OPINIONS: The Gemara (25b) rules in accordance with the view of Rebbi Yosi who states that a person is permitted to plant a tree inside his property next to his neighbor's Bor. The only case in which Rebbi Yosi prohibits one from doing a potentially damaging action in one's own domain is when the action involves "Girei Dilei" (lit. "his arrows") which directly cause damage to the neighbor's property.
If the Halachah follows the view of Rebbi Yosi, does this mean that the Halachah does not follow the Mishnah earlier (17a) which states that a person must distance his Bor and other potentially damaging agents, such as a water channel (Amas ha'Mayim), olive residue (Gefes), or seeds (Zera'im), from his neighbor's property? That Mishnah does not seem to follow the view of Rebbi Yosi, since those objects are not "Girei Dilei."
In the case of a person who digs a Bor next to his neighbor's property, the Gemara (17b) clearly states (according to Rava) that it is considered "Girei Dilei," because when he pounds the earth with the shovel as he digs, he weakens the earth of his neighbor's land. What, though, is the Halachah in the case of the other damaging agents, which do not involved striking the ground with a shovel?
Also, if a person does cause damage to his neighbor's property in a manner which is "Girei Dilei," is he obligated to pay for the damage or not? (See Insights to 17:3.)
(a) RASHI (22b, DH Leima) writes that "many of the rulings of the Mishnah" follow the opinion of the Rabanan (who argue with Rebbi Yosi). Rashi seems to refer to the cases in the Mishnah (17a), except for digging a Bor (which the Gemara calls "Girei Dilei," as mentioned above).
Rashi in a number of places defines "Girei Dilei" as "arrows that damage while coming directly from the body of the person doing the damage" (22b, DH b'Girei; 25b, DH b'Girei; Bava Metzia 117a, DH Modi). According to this explanation, an act can be considered "Girei Dilei" only when the damage occurs immediately. Accordingly, none of the cases in the Mishnah qualify as "Girei Dilei" and the Mishnah must be following the opinion of the Rabanan.
With regard to the obligation to pay for damages, since Rashi defines "Girei Dilei" as arrows that come from the person who is damaging, it seems that Rashi equates a case of "Girei Dilei" with actually shooting an arrow -- in which case the person is "Adam ha'Mazik" and is obligated to pay. If the damage was done in a manner which is only a "Gerama" (such as the case on 22b, where the person merely places the marten in a position that readied it to do damage), then obviously he would be exempt from paying for the damage.
(b) TOSFOS (22b, DH Leima) cites RABEINU CHANANEL who writes that the Mishnah earlier (17a) indeed follows the opinion of Rebbi Yosi. He explains that "Girei Dilei" means that the process of the damage begins at the time that the person does his act (even though the full damage does not occur until later).
(This is also the opinion of the RIF, the GE'ONIM, and RABEINU YONAH (18b).)
According to Tosfos, if the damage does not begin immediately, then it cannot be considered "Girei Dilei." This is why Tosfos (17b, DH Marchikin, and 18a, DH Deika) writes that Rebbi Yosi would permit -- even according to Rava -- placing Gefes or Melach (salt) near the border of one's property before the neighbor builds a wall in his own property. Rava only prohibited doing so according to the Rabanan who argue with Rebbi Yosi.
The Rishonim ask why planting Zera'im next to a neighbor's property should be considered "Girei Dilei" according to this opinion. The damage that they cause occurs only when their roots grow and soften the dirt and thereby weaken the base of the wall (as the Gemara on 19b says). RABEINU YONAH answers that the damage is not from the growth of the roots, but rather it results from the nourishment that the Zera'im draw from the land and thereby weaken the earth. This damage begins immediately when the seeds are planted.
The RAMBAN cites this explanation and asks that the answer does not suffice, because even the nourishment that the roots draw from the ground does not begin immediately.
(c) The RAMBAN (17b, and in the end of Kuntrus Dina d'Garmi) writes that whenever the damage results directly from the action that the person performed it is called "Girei Dilei," even if the damage does not start until a later time. This the reason why the Zera'im are also considered "Girei Dilei" even though they start to soften the ground only after they are planted. Similarly, "Girei Dilei" prohibits one from placing Gefes or Melach near the border of one's property even before the neighbor builds a wall on his side of the border, since the damage to the wall will result from the Gefes having been placed in this location.
The only case that is not a case of "Girei Dilei," according to the Ramban, is the case of a tree planted next to a neighbor's field. This is because the damage that the tree causes does not come from the tree itself, but rather it comes from the roots, and the roots were not planted by the owner in a place in which they could cause damage.
With regard to the obligation to pay for damages, the RA'AVAD (cited by the Ramban and the Shitah Mekubetzes here) rules that one is exempt from payment, because all of the damages of "Girei Dilei" are considered "Gerama," as the Gemara states in the case of the marten. However, the Ramban himself compares the cases of "Girei Dilei" -- such as placing a water pit near a neighbor's wall -- to placing a piece of coal on another person's clothing which slowly destroys the clothing. In the case of a coal, it is obvious that the owner of the coal is obligated to pay for the damages. The same applies in the cases of "Girei Dilei." He provides support for this from the fact that the Gemara describes the damage as "Girei," meaning "his arrows," for which a person is obligated to pay (as mentioned above according to Rashi).
2) DAMAGE CAUSED BY SHAKING FLAX
QUESTION: The Gemara relates a case in which certain people were shaking flax in order to remove the chaff, and the chaff was damaging other people. Ravina ruled that they are permitted to shake the flax, and it is the obligation of the bystanders to distance themselves from harm.
Mereimar argued with Ravina's ruling and said that one is prohibited from shaking the flax if it damages others, because it is his action that is causing the chaff to fly off, just as one who winnows on Shabbos is liable for the act of separating the chaff from the grain.
The Gemara questions Ravina's ruling and asks why this case differs from the case of a spark that flies out due to a hammer-blow and causes damage, for which the person hammering is obligated because of "Esh." The Gemara answers that the difference is that the person hammering wants the spark to fly out (so that it not ignite his house, as Rashi explains), whereas the person shaking the flax does not want the chaff to fly outwards.
Why should shaking the flax not be considered an act of Esh? The one who shakes the flax should be obligated, just like a person who places a stone in a place where the wind can blow it down is obligated for damages because of Esh. The person who places the stone there, or the person who lights a fire, does not want the stone to fall or the fire to spread, and yet he is obligated to pay for damages. (TOSFOS)
Similarly, when a person is cutting wood and a piece of the log flies out and causes damage, he is obligated for damages (because of "Kocho" and "Adam ha'Mazik"), since his action causes the damage, even though he wanted the wood to remain where it was so that he could use it. (RAMBAN in Kuntrus Dina d'Garmi)
(a) TOSFOS (DH Zika) explains that a person is liable for damages of Esh only when he lights the fire by himself. If the fire is lit only with the help of the wind, he is not liable for Esh according to Ravina.
How does this explain why a person is liable for leaving a stone in a place where the wind will blow it? The stone fell down and caused damage only with the help of the wind!
The MAHARSHA explains that the owner of the stone accomplished what he intended to do by placing the stone where he left it. Therefore, he is comparable to a person who made a fire entirely by himself. However, in the case of the flax (and in the case of a person winnowing), the person shaking the flax had no intention to leave the chaff in the air, but rather he intended that it fall to the ground where it no longer could be blown away by the wind. Since he did not accomplish his goal with the chaff before the chaff was blown away and caused damage, it is not comparable to Esh. This also seems to be the intention of the RAMBAN (in Kuntrus Dina d'Garmi).
(b) The TOSFOS RID writes that the damage caused by the chaff indeed is considered Esh. Nevertheless, Ravina exempts a person from paying for damages caused by his fire when the people who were damaged were aware of the fire and were able to protect themselves. Therefore, if a person places a stone atop his roof next to his neighbor's property in a place where his neighbor certainly sees it, then it is his neighbor's obligation to move his objects away so that they will not be damaged if the stone falls (according to Rebbi Yosi, who maintains that it is the Nizak's obligation to protect himself). The Torah obligates a person for damages caused by his fire (or stone) only when the person who was damaged was innocently walking through Reshus ha'Rabim and was not aware of the fire or the stone such that he could protect himself from it, or when a neighbor was not home (when a fire was lit in the adjacent property), or when it was impossible for him to protect his grain from the approaching fire.
A possible source for the logic of the Tosfos Rid may be the Halachah that one who digs a pit is exempt from damages caused to a person who falls into the pit. According to Rashi (Bava Kama 5b, DH v'Chulu; see also Tosfos, Bava Kama 5b, DH Ki, and Insights there, and see MESHECH CHOCHMAH, Parshas Mishpatim), the reason for this Halachah is that the person (the Nizak) should have been cautious and avoided the pit (even when the pit is in Reshus ha'Rabim).
The same reasoning may apply to damages caused by one's Shor. A person is exempt from damages of Shen and Regel in Reshus ha'Rabim, because the people in Reshus ha'Rabim should expect the Shor to cause such damage and thus they should protect themselves from it. However, one is liable for damages done in Reshus ha'Rabim by his Shor ha'Mu'ad (through Keren), because people do not know that the Shor is prone to cause damage in such a manner since they do not know that the Shor is a Mu'ad. (The owner of a Shor Tam pays Chatzi Nezek because of a Kenas.)
(c) The RAMAH (#107) explains, like the Tosfos Rid, that damage caused by the chaff is comparable to damage caused by Esh. However, he explains that according to Ravina a person is liable only for damages caused by a fire ignited in his domain when he wanted the fire to spread, or when the fire spread by itself without the assistance of the wind. When he lights the fire outside of his domain, he is always obligated to pay for damages, because Rebbi Yosi exempts a person only when he was acting in his own domain.