The gemarah asked ; For what purpose is water compared to blood , according to
Chizkyia " and the gemarah answers to teach us that Kodshim does not require
Hechsher to become tomei ?
Two questions:
(a) How does Rav Avouho knows this din, since he already needs this comparaison for Heter Hano-o for blood ?
(b) Why isn't the gemarah asking this obvious question, like it normally does"
"Ve-idoch ? " ?
I must be missing something, since my obvious question didn't bother Rashi or
Tosfos ?
(a) It seems to be clear from TOSFOS (22a, DH Mah Mayim) that Rebbi Avahu derives both Dinim from this verse. Perhaps he learns the Heter Hana'ah from the Hekesh of blood to water, and he learns that the blood of Kodshim is not Mach'shir from the fact that the Torah uses the phrase " Tishpechenu k'Mayim" and not "Tipol k'Mayim." From there we derive that that which is "Nishpach" like water is Machshir, and that which is not Nishpach is not Machshir.
I saw that the EVEN HA'OZER says that both Dinim are derived from the verse, since "Ein Hekesh l'Mach'tzeh," and blood is compared to water also with regard to the Din of blood of Kodshim (this needs further analysis, because that Din is not derived from the Hekesh per se, but from a Limud).
(b) It seems that this was simple to the Rishonim and did not have to be discussed. (Alternatively, it is not possible to prove that Rebbi Avahu agrees to the Halachah of Rebbi Yochanan, and it is possible to say that he argues that Dam Kodshim is Machshir.) According to the Even ha'Ozer, who says that "Ein Hekesh l'Mach'tzeh" applies here, it is obvious why the Gemara does not ask, "v'Idach".