פירוש על שמות 22:5

כי תצא אש IF A FIRE GOETH FORTH — i. e. even if it goeth forth (extends) by itself (Bava Kamma 24b) from the field in which it has been lit into another persons field.

כי תצא אש, If fire escapes (from someone's private domain), etc. The Torah means that even if one lights a fire in one's own private yard such a person is liable for damage caused by such fire outside his domain if the blaze was such that the owner should have foreseen it could extend beyond his own fence. If an individual lit a fire in his private yard which could not be expected to extend beyond his own boundaries he is free from responsibility if it happened unexpectedly. Such an accident would have to be considered an act of G'd (compare Baba Kama 61). When the Torah speaks about ומצאה קוצים, that the fire "finds thorns," it means that even if there were no other inflammables in its path, the fact that the stack of corn is consumed by fire results in the person starting the fire having to make repayment, שלם ישלם. He will have to pay for either the stack of corn or the field in which the corn is still growing, as the case may be. He will even have to pay for the stalks though the latter were a contributing cause to the corn being burned. All of this applies only if the fire "found" the thorns. If a person other than the one who started the fire contributed to its spreading, then that second person is liable for the entire damage which occurs as a result.

כי תצא אש, the one lighting the fire was negligent in not taking sufficient precautions that the fire be confined to his own property assuming no extraordinary wind is blowing.