Chullin 9: Torah Scholars' Skills, What is Permitted

Rav Yehuda quotes Rav, saying that a Torah scholar must learn three skills: writing, so that he can write texts for several occasions, ritual slaughter, and circumcision.  Rav Hanna bar Shelamya quotes Rav as well.  He says that Torah scholars must also be able to tie the knot of the tefilin, to recite the groom's blessing by heart with traditional intonation, and to tie the tzitzim. 

Rav Huna explains that the is an presumption of permissibility with animals.  Until they are selected for shechita, they are assumed to be permissible. Once selected, they are presumed to be permissible until they become treifa.  Even if there is a flaw, they are presumed permitted until checked and possibly declared ritually impure.  

The rabbis remind us that there is a difference between something that is not permitted because of danger and something not permitted because of ritual impurity.  If we are concerned about snake venom in a small hole bored into a fig, for example, that is a concern about danger.  We are reminded that we continue to wash our hands before eating based on the priestly requirement of hand washing to ensure that they are not transmitting ritual impurity.