How can rashi on 51a say that tumah kalah makes kiyotzeh bah and refering to being metamah another ochel MEDERABANAN when the gemarah is trying to make a kal vechomer on a dearaisha level to not need hechsher. a derabanan din won't break a din dearaiasa.
shmuel freedland, NY, USA
Baruch she'Kivanta! Your question is asked by the Aruch la'Ner.
1. The Sefer Mei Nidah answers that in fact the Rabanan who disagree with Rabbi Yochanan ben Nuri knew that his Kal va'Chomer is not a perfect Kal va'Chomer ("Kal va'Chomer Gamur"), because even Rebbi Yochanan agrees that Min ha'Torah one requires Machshavah. It is only mid'Rabanan that Rebbi Yochanan learns from this Kal va'Chomer that it is fitting to be stringent in this matter and say it is Tamei even without Machshavah. (Hence, since the Chachamim instituted that food can be Metamei food, it is now possible to learn a Kal va'Chomer that it will be Metamei even without Machshavah.)
2. The Nachalas Yehoshua answers that Rashi's opinion is that one can reject a Limud of a Din of the Torah even with a Halachah that is only mid'Rabanan. Rashi also maintains this opinion in Chulin (136b, DH Bifnei). However, Tosfos there (DH Lifnei) disagrees with Rashi and writes that one cannot nullify a Gezeirah Shavah d'Oraisa because of a Din d'Rabanan.
Kol Tuv,
Dovid Bloom