Hello, on daf 14a there's a discussion about why it was decreed that a person who eats rishon or sheni becomes a sheni, and the answer is that it prevents the scenario where he's eating tamei food and then drinks liquid terumah, thereby making the terumah tamei, in his mouth.
But a simpler question is why does it need to be this unique scenario where the terumah encounters the tamei food in his mouth? If the person is a sheni, then even if he has no food in his mouth doesnt the terumah liquid become tamei just by contacting the person himself?
Additionally why would we decree on all of yisrael for a scenario that only impacts kohanim (since they are the only ones eating terumah), if the concern is only about the inside the mouth contact.
Thank you!
Josh Danziger
1) Rashi DH uPasil gives us a hint when he writes that the Torah warns us to keep Terumah pure. He writes that therefore Chazal made a gezera that someone who eats tamei food should not be frequent with Terumah. Rashi is explaining what the most necessary protection for Terumah is. We want to make sure that only people who are careful only to eat pure food, should have regular contact with Terumah. That is why the gezera specifically involves a person who ate tamei food, because it is precisely this sort of person that we wish to keep away from Terumah.
2) The Rambam writes, in his introduction to Seder Taharos printed at the beginning of Mishnayos Taharos (before the beginning of Maseches Keilim. In the older edtions of the Mishnayos it is printed from sefer Maleches Shlomo #25) that a person can be Av Hatumah or Rishon leTumah but he can never be sheni leTumah, neither Min Hatorah nor miderabanan. There is only one way he can be sheni. This is if he eats or drinks tamei food or drink, but this will only be tamei miderabanan. Before the gezera was made, a person could not be metamei liquid just by touching it.
Joshua, wishing you a Healthy Winter!
Dovid Bloom