The Mechaber and the Rema assume that a father does not perform the milah himself, but rather appoints another (the mohel) to do the mitzvah on his behalf. Yet a mishnah (Kiddushin 29a) lists circumcision among the mitzvot that are incumbent upon a father to do for his son. Clearly it is preferable that a father circumcise his child himself; if so, why shouldn’t he serve as sandak as well?
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In Responsa Divrei Malkiel (Rabbi Malkiel Zvi Tannenbaum, 1847-1910, Vol. 4:86), we find an explanation of the role of the sandak at the brit. The sandak, Rabbi Tannenbaum writes, secures the feet of the infant, holding them in place, and positions the milah toward the mohel. If so, the sandak actually plays a significant part and assists the mohel in the physical part of the mitzvah. It is as if the two of them have done the mitzvah together.
Rabbi Tannenbaum compares sandika’ot at the brit to other instances where two collaborate in an act. (This is relevant for a brit milah performed on Shabbat and whether collaboration exempts one from full halachic responsibility.) For instance, the mishnah (Shabbos 92b) states: “If one carries out a loaf [on Shabbat] into the public domain, he is liable. If two carry it out [into the public domain], they are not culpable. If, however, one [alone] is unable to carry it out on his own and thus two carried it out, both are liable; however, Rabbi Shimon exempts them.”
Thus, we see that an act normally done by one person – or able to be done by one person – but is in fact done by two together renders neither one liable for that act (if the act is forbidden). (Rabbi Tannenbaum notes that an exception to this rule is found in the Gemara [Makot 20b], derived from the Leviticus 19:27. A makif [one who shaves another’s peyote] and a nikef [one who allows another to shave him] are both liable even though each is doing only half an act.)
Now, regarding a mohel and sandak, we find that the Mechaber (Yoreh Deah 266:14) issues the following warning: “Care is to be taken that two mohalim not perform the brit together on Shabbat, [i.e.,] that one circumcises [milah] and the other reveals the crown [priyah]. Rather, the one who circumcises shall himself [also] do the priyah.”
The implication of the Mechaber’s statement is that since two would be performing the act together, it is possible that neither is fully accomplishing the act on his own, thereby leaving each in violation of Shabbat. The Rema (ad loc.), while essentially not agreeing with the Mechaber’s logic – since he argues that the avodah in the Beit Hamkidash on Shabbat was performed by numerous kohanim and yet was not considered a violation of Shabbat – cites a source that prohibits the division of the brit between the mohel and sandak, even though he himself would find no such reason to prohibit it.
Rabbi Tannenbaum alludes to this halacha of the Mechaber and applies it similarly to the roles of the mohel and the sandak at the brit. If we consider that the sandak plays a significant role in the actual milah, then the same problem arises at any milah performed on Shabbat.
He offers two possible solutions: One, since the mohel in the course of the milah is allowed to inflict a wound in the act of circumcision, he instantly overrides the violation of Shabbat. Second, the sandak actually does not aid that much in the performance of the milah. Therefore, it is considered as if the mohel has the ability to override Shabbat but the sandak does not. Since the mohel already overrode Shabbat by inflicting the actual wound, the sandak is not liable in such a case. Nevertheless, when the sandak holds the child, he is considered as having a part in the mitzvah of the milah. Additionally, he is considered to have the same status as a kohen who offered the ketoret.
Rabbi Tannenbaum cites the Chatam Sofer (Responsa Orach Chayyim 159) – which he notes that he saw only after he wrote his own responsum. The Chatam Sofer basically arrives at the same conclusion, that the sandak accomplishes two mitzvot: first, he holds the infant and second, he plays an ancillary role in the act of the circumcision and as such precedes the mohel in importance as relates to the mitzvah. (See the Chatam Sofer, who has a different source, the sefer of Rabbenu Peretz, actually cited by Maharil found in Rema , Y.D. 265:11, who cites Midrash Rabbah to Parashat Vayera, from where we derive the comparison of the sandak to the kohen offering ketoret, as opposed to Zohar – Parashat Pikudei – we cited at the outset.)