Menachot 56 - Adding Blemish to a Blemish

A firstborn animal must be given to a kohen. If it develops a permanent blemish, it belongs to the owner for consumption, but it is forbidden to make a blemish on purpose. If an animal is sick and the only remedy is to let blood, then there are four opinions on the permissibility of it. Rabbi Meir allows blood-letting, provided that this is done in a place where it does not cause a permanent blemish. The Sages allow to make a blemish, but not to slaughter based on it. Rabbi Shimon allows even to slaughter it, and Rabbi Yehudah absolutely forbids to led blood, even if the animal will die if left untreated.

The question here is adding a blemish to an already blemished - by being sick - animal. There are two similar situations where everyone agrees that it is forbidden. If one bakes a leavened flour offering, he is adding to leavening, and is thus liable. If one further castrates a male animal, adding to an existing castration, he is liable. In the case of a firstborn, the strict rulings are based on the Torah verse "there shall be no blemish," and the lenient ones - on "it shall be unblemished to find favor," meaning that if it is already blemished - then it does not find favor.

Art: Winslow Homer The Sick Chicken