As we have learned, the mitzva to perform circumcision overrides the prohibitions of Shabbat. This rule does not fit into the usual rubric of aseh doheh lo ta’aseh – that a positive mitzva overrides a negative one – since Shabbat is not simply a negative commandment; it is a positive commandment, as well. For this reason the Gemara on today’s daf brings a number of different opinions offering different sources for this law. The Gemara also discusses other cases where positive and negative commandments stand in contradiction and rulings must be made based on setting priorities. For example, the mitzva of circumcision will be performed even if it involves cutting off a leprous sign on the child’s foreskin, but someone suffering from leprosy may not perform the Temple service even though it is a mitzva. The Gemara explains:
Rather, Rav Ashi said that this is the reason that leprosy does not override the Temple service: Where do we say that a positive mitzva overrides a negative mitzva? It is in cases like circumcision in a case of leprosy, or alternatively, tzitzit (ritual fringes) and kilayim (diverse kinds of wool and linen), as at the time the negative mitzva is uprooted, the positive mitzva is fulfilled in the very same action, e.g., when the ritual fringes are woolen and will be attached to a linen garment, a prohibited mixture is created. However, here, in the case of a person afflicted with pure symptoms of leprosy cutting off his symptoms to enable his involvement in the Temple service, it is different, at the time the negative mitzva is uprooted, the positive mitzva is not yet fulfilled, as cutting off the symptoms is only a preliminary action that enables him to serve. In that case, the positive mitzva does not override the negative one.
The cases of ritual fringes and diverse kinds cited here are not classic examples of positive mitzvot overriding negative mitzvot, because both cases mentioned here are derived from a particular source. The halakha in the case of leprosy and circumcision is derived from an a fortiori inference, and the halakha in the case of ritual fringes and diverse kinds is based on a juxtaposition of verses. Nevertheless, the following principle learned from these cases applies to other laws: The positive commandment overrides the negative commandment only when the fulfillment of the positive commandment is simultaneous with the violation of the negative commandment (Ramban; see Me’iri).