Up until this point, Massekhet Eiruvin has discussed the rules of the types of walls that are necessary to create a reshut ha-yahid – a private domain – that will allow carrying on Shabbat. The third chapter, Bakol Me’arvin, introduces another essential ingredient necessary for the eiruv to work – food. In order for families or groups of people to be considered united in a private domain, they need to be partners in enough food for two meals. The Mishna (26a) rules that any food can be used to create this eiruv, except for water and salt.
The Jerusalem Talmud offers two reasons for the exclusion of water and salt. The first reason is the obvious one. Since these two items, while edible, do not offer any real sustenance, they cannot be considered food. The second reason is that both of these items hint to destructive punishments that appear in the Bible. Water destroyed the generation of the flood, and the city of Sodom was turned into salt.
Aside from eiruv, salt and water are excluded from other halakhot, as well. For example, when a farmer finds that he has so much Ma’aser sheni (the second tithe – which is supposed to be eaten within the walls of Jerusalem) that he cannot transport it all, and perhaps he cannot eat it all during his visit to the holy city, he is allowed to redeem the produce and take the proceeds to Jerusalem, where he can purchase food items. Among the edible items that cannot be purchased are water and salt. This is based on the passage ( 14:26) that seems to permit the farmer to purchase “all that [his] heart desires” with the money, but then enumerates cattle, and wine, closing, again with “whatever [his] soul requests.” This passage is interpreted by ben Bag Bag in a baraita quoted by the Gemara, to allow even for the purchase of animals whose purchase price includes wool or a valuable hide, but only when the central purchase is a food item.
Yohanan ben Bag Bag lived in the time of the Mishna, during the period of the destruction of the Second Temple. While he has few statements that appear in the Talmud, it is clear that he was respected by his peers as a scholar, to the extent that Rabbi Yehudah ben Betaira says about him that he was expert in the secrets of the Torah. Some say that he was from a family of converts, which explains the odd name – Bag Bag. Tosafot say that the numerical value of the letters of “bag” equal five, the value of the letter heh added to Avram’s name when he become Avraham – the father of many nations. Others explain that “bag” is an abbreviation of ben gerim – the son of converts.