Our Gemara establishes a principle that a person cannot be held responsible for an ones – an occurrence beyond his control – and even if he accepts responsibility for ones, that only will include relatively common occurrences. An unsa d’lo shakhiah – something beyond the person’s control that is totally unexpected – will not be his responsibility, even if he accepted responsibility for situations of ones.
The cases that the Gemara uses to illustrate this point require an understanding of the reality on the ground in the place where the sages lived. In Babylonia there was very little rainfall, and the vast majority of water used for irrigation came from the large rivers that flowed through the country. Already during ancient times, the local government initiated projects to build large irrigation canals. Some of these canals were so wide that they were viewed as small rivers, particularly at the points where they received water directly from the Tigris or the Euphrates, and they served not only for irrigation but also for travel and transport on small boats that plied them as waterways.
In the first case presented by the Gemara, someone sold a field to another person, accepting upon himself any ones that might take place. After the purchase was completed, the local government announced plans to dig a canal that would go directly through that field. The case was brought to Ravina who ruled that the seller, who had accepted any ones that might occur, needed to return the purchase price to the buyer. An objection was raised that this was an unsa d’lo shakhiah, and that he could not have intended to offer a guarantee against such an unlikely possibility, a position that was upheld by Rava.
A second case that is brought tells of an agreement made between Rav Papa and Rav Huna brei d’Rav Yehoshua and a group of sailors who agreed to transport their merchandise across the canal known as Nehar Malka – the royal river. The sailors agreed to accept responsibility for any ones that might occur. In fact, the canal was dammed up and they could not transport the goods. Rav Papa and Rav Huna the son of Rav Yehoshua insisted that the sailors hire donkeys to transport the goods, but Rava insisted that the sailors could not be held responsible, since they would never have accepted responsibility for an unsa d’lo shakhiah.
Nehar Malka, which connected the Tigris and the Euphrates, was one of the largest canals in Babylonia. It would certainly have been unusual for this major artery to have been closed, making such an event an unsa d’lo shakhiah.