Rashi says (Umah Ochel Habah Machmas T'Vul Yom) that in the Kal V'Chomer that R' Yosi should have made, that the way the Gemara can say Amrat Oseh Revei B'kodesh - comes from the Kal V'chomer of Mechusar Kippurim and if that's the same place where Abah Shaul got his din, then why can't R' Yosi learn it out from the same place?
Tzvi Levi, Manalapan, USA
Do you mean to ask why does the Gemara say that Rebbi Yosi should have learnt from Ochel ha'Ba'ah Machmas Tvul Yom if this Limud also needs to come onto the Limud from Mechusar Kipurim?
Tosfos (30a) asks this question (30a DH Shlishi). Tosfos answers that whereas in the Limud Rebbi Yosi actually made we need to learn out from Mechusar Kipurim that a Shlishi is Tamei from Tvul Yom, in the Limud the Gemara suggests, we only need to learn from Mechusar Kipurim that Ochel ha'Ba'ah Machmas Tvul Yom is Posel Kodesh.