We have already learned that a mavoy (alleyway) is the area into which a number of courtyards open, which allows access to the reshut ha-rabim – the public domain. Carrying in a mavoy will be permitted on Shabbat if a symbolic board (a lehi or a kora) is placed at the entrance to the reshut ha-rabim, and if all of the residents have shared ownership of food, which figuratively joins them together.
If someone wants to ensure that he will be able to carry in a mavoy without entering into negotiations with his neighbors, the Mishna on our daf teaches that he can take a barrel of food that belongs to him and announce “this is [for the eiruv] for all the residents of the alleyway.” This arrangement works by virtue of the rule zakhin l’adam shelo befanav – that even without someone’s knowledge, another person can engage in activities that benefit him. The Mishna teaches that another person needs to play the role of the agent who is acting on behalf of those people who are not aware that the transaction is being done for them. This agent can even be the adult children of the person who is making the eiruv, or his wife or his Jewish slaves.
These rules are not unique to eiruv. In fact all of the normal rules of zakhin – of acting on behalf of someone else as their agent – need to be followed, the most basic of which is that one person cannot do it on his own; he needs another person to play the role of the “purchaser.”
An interesting disagreement turns up regarding who can act as the agent. Although the Gemara permits someone’s adult children or his wife to play that role, some commentaries argue that if family members are supported by the head of the household, according to the halakha all of their income automatically belongs to the father. In that case, perhaps they should be viewed as agents of the father and cannot represent the other side in what is, in essence, a financial matter.