The Mishnah (5:2) records a debate regarding a bechor that requires bloodletting. The issue is that one might, as a result of the procedure, create a mum (blemish) that invalidated the bechor from being offered as a korban. Doing so deliberately is prohibited. A bechor with a mum is nevertheless given to a kohen who would be allowed to eat it.
R’ Yehuda forbids any bloodlettings even at a location where a mum will not occur. Rashi explains that due to one’s innate concern for his property, allowing the procedure in safe areas, may result in them being perform in other areas as well. The Chachamim permit it, provided that one does not cause a mum in the process. If one does, they would need to wait for another mum to develop in order to permit its consumption. R’ Shimon however is not concerned even if a mum is created, since that was not one’s intention when doing so.
The Gemara cites a Beraita that also includes the opinion of R’ Meir who maintains that one can only perform the procedure in a location that will not cause a mum. The Chachamim in the Beraita maintain that one could perform it anywhere, yet one could not slaughter the animal based on that mum.
Rashi explains that the debate in the Beraita is regarding a case where the animal will not recover from its condition without bloodletting and will otherwise likely die. The animal is therefore, at this point, defined as a ba’al mum.*Consequently the debate between R’ Meir and the Chachamim is whether one is allowed to inflict a mum on a bechor that has a pre-existing mum*.
The Gemara (33b) explains that the debate hinges on the understanding of the following pasuk. “When a person offers a korban Shelamim to Hashem, to fulfil his neder or nedava, with cattle or flock, they should be tamim… all blemishes shall not be on it.” R’ Meir understands that “kol mum”, the addition of the word “all”, includes adding mumim to a korban that has one already. Chachamim however note that it is preceded by a reference to tamim, unblemished animals, thereby limiting the prohibition.
The Beit Efrayim (YD 75) understands that at the core of the debate is a dispute around the nature of the prohibition. The Chachamim understand that the prohibition is for one to invalidate a korban from being offered on the mizbeach. The act of inflicting a mum is tantamount to stealing from the hekdesh. If the animal had a pre-existing mum and is already disqualified, then there is no biblical prohibition against inflicting a mum. R’ Meir however understands that the prohibition relates to the sanctity of the animal. Inflicting a mum on a sanctified animal is itself an issue. Consequently it is irrelevant if it has mumim already or not.
One point to clarify, the Tosfot (33b s.v. “baal”) explains that even thought we have learnt that inflicting a mum on a baal mum is permitted on a biblical level, it is still prohibited on a rabbinic level. They only permitted it in this context of blood letting.
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