The Gemara tells a story about a brit mila (circumcision) that was taking place on Shabbat, where the hot water that had been prepared – and were essential to doing the brit properly – spilled. Rabba ordered that more water be brought from the house into the courtyard, but his student, Abaye, argued that a proper eiruv had not been made. Faced with that issue, Rabba suggested that a non-Jew be asked to bring the water.
Asking a non-Jew to perform a forbidden act on – Amira la-Akum – is, itself, Rabbinically forbidden. The Rosh explains that Rabba suggested making use of the non-Jew only in this case of a circumcision. Since a brit mila has the unique status of pushing aside Shabbat (see Massekhet), it is logical that we would permit an act forbidden by the Sages, as well.
Abaye said: I wanted to raise an objection against the Master, Rabba, but Rav Yosef would not let me do so, as Rav Yosef said that Rav Kahana said: When we were in Rav Yehuda’s house, he would say to us when we were presented with a difficulty: With regard to a Torah law, we first raise objections and then we perform an act, i.e., if someone has an objection to a proposed action, we must first clarify the matter and only then may we proceed. However, with regard to rabbinic laws, we first perform an act and then we raise objections.
After the water had been brought and the circumcision performed, Abaye was asked to present his question. He asked why completing the ritual to purify someone who had become tame (ritually impure) – which is forbidden on Shabbat by the Sages – cannot be performed even if it is necessary to perform a mitzva (e.g. to sacrifice and eat the Passover sacrifice), yet in our case, asking a non-Jew to bring water for the brit is permitted?
Abaye’s question is particularly powerful because missing the opportunity to participate in the Passover sacrifice was punishable by karet (being cut off from the community), which is also the punishment for neglecting the commandment of circumcision. If anything, we would have anticipated that there is more reason to try and accommodate the person who wants to bring the sacrifice on Passover, since he has to do it on one particular day – the 14th of Nissan – while a child who is not circumcised on the eighth day can have the brit later on, as well.
The Gemara’s response to Abaye’s question is that we distinguish between an “active” Rabbinic prohibition and a “passive” one. In our case, Amira la-Akum is passive, so we are more comfortable pushing it aside when necessary.