Bekhorot 10: Food Impurity and Shechitah Practice

A debate between Rabbi Shimon and Rabbi Meir, as to whether a first-born donkey is permitted for benefit. Whether the animal is decapitated is a relevant element to the disagreement. Plus, the question of food impurity -- that which isn't forbidden for benefit is food, and vice versa, but then it can become impure. But then what about meat and milk that are cooked together, and prohibited for benefit? Because that's not Rabbi Shimon's opinion. The system of food impurity seems to be internal to itself. Also, the case of slaughtering a first-born animal for practice in slaughtering. [Who's Who? Nimos (and Rabbi Yehoshua HaGarsi)] Plus, Rashi's comment, as well as that of Rabbenu Gershom.