Nedarim 68 - 74
The Limits of Agency
A person can appoint an agent to act in his behalf, and the agent's action is considered as his own. We have already encountered this broad rule of shlichut -- agency -- in our own mesechta (36b) regarding someone appointing an agent to tithe his produce in his behalf. In other parts of the Talmud we learn of the ability to effect marriage and divorce through an agent.
Can a Jew appoint an agent to sit in a succah for him on the holiday of Succot, or to put on tefillin in his behalf? This question has been raised by both early commentators (Tesofot Ryd in Kiddushin) and later ones (Ketzot Hachoshen 182:1). Although they differ in their explanations, there is a clear consensus that one cannot fulfill such a mitzvah through an agent.
The Ketzot bases his explanation on our gemara. A man has a right to cancel a vow made by his wife if he does so during the day that he heard it made. The Sage Romi bar Chama raised the question as to whether he must actually hear the vow or whether he can declare a general cancellation even if he did not hear it. The gemara attempts to resolve his question by citing a beraita dealing with the issue of a husband appointing an agent to cancel any vow his wife will make while he is away. Rabbi Yonatan's opinion is that the agent's annulment is valid. Even Rabbi Yoshiah who disagrees does so on the basis of his understanding of the Torah passage expressly limiting this power to the husband himself. There is a consensus, however, that agency should apply in theory to cancellation of vows. This seems to suggest that it is not necessary for the husband to actually hear the vow being made.
Even though this proof is rejected, it does raise an interesting point. How can we conclude, based on the fact that the husband is away, that there is no need for him to actually hear the vow? Perhaps he does need to hear the vow, but here he has appointed an agent to hear the vow in his behalf?
Rosh explains that although he can appoint an agent to cancel vows, it is impossible for the agent to serve in his behalf as the hearer of the vow. This is because agency is effective only in regard to acting, not to a passive experience like hearing.
The same is true in regard to mitzvot notes the Ketzot. When an agent performs the actions required for tithing, marriage or divorce, it is considered as if the one who appointed him is doing them. In regard to tefillin, however, the Torah commanded you to put tefillin on your arm and your head. When an agent puts them on his arm and his head it is truly considered as if you did the placing, because that is action. But you have placed it on his body, not yours, and have therefore failed to perform the mitzvah.
Condition of the Condition
If a man makes a vow and states that he is doing so on the understanding that his friend can cancel it, does that friend have the power to do so?
Ran cites an opinion of some commentators that the friend does have the power of cancellation. They base this on the statement in our gemara of Rabbi Pinchas, that the reason a husband has the power to cancel his wife's vows is that when a married woman makes a vow, we assume that she does so with the understanding that her husband will approve. If this assumption is sufficient for us to relate to a woman's vow as being conditional on her husband's approval, then it makes sense that a person's explicit statement regarding the power of cancellation for his friend should also be considered as making that vow vulnerable to cancellation.
This is an incorrect comparison, says Ran. If we literally interpret Rabbi Pinchas' statement as meaning that a wife's vow is made with a formal condition that her husband approve, then there should be no requirement for the husband to use a particular text for his cancellation as is indicated later (77b). Rather, Rabbi Pinchas is merely providing a reason that the Torah explicitly delegated to the husband the power of cancellation: Since a woman presumably does not wish to make a vow which will not find favor with her husband, the Torah granted him the power to make a formal cancellation. Since the Torah does not mention such a power for one man in regard to another, he does not have the ability to make such a cancellation even if the vow-maker expressly wished to extend that power to him.
This is so, concludes Ran, only when the vow-maker wishes to delegate the power to cancel. If he makes an express condition, however, that his vow is dependent on his friend approving it, then the vow never takes effect if his friend expresses disapproval in any form, because the vow was made conditional on his approval.