1) AN AMORA WHO ARGUES WITH A TANA
QUESTION: The Gemara quotes Rebbi Yochanan who says that the Torah requires Perishah only for the Avodah of Yom Kippur, but not for preparing the Parah Adumah. The Perishah of Parah Adumah is merely a "Ma'aleh b'Alma." The Gemara asks that earlier (2a) Rebbi Yochanan said that both Yom Kippur and Parah Adumah require Perishah. The Gemara answers that the earlier statement of Rebbi Yochanan was said in the name of his teacher, Rebbi Yishmael. Rebbi Yochanan himself, however, maintains that only Yom Kippur needs Perishah.
How can Rebbi Yochanan, an Amora, argue with Rebbi Yishmael, a Tana? (TOSFOS in Kesuvos 8a, DH Rav, proves that Rebbi Yochanan was a full-fledged Amora and cannot argue with a Tana, unlike Rav who had the status of a Tana and was entitled to argue with a Tana.)
(a) TOSFOS (DH d'Rabei) explains that the Gemara (4a) cites proof from a Beraisa for Rebbi Yochanan's opinion that the requirement for Perishah is derived from the Milu'im. The text of that Beraisa in our edition of the Gemara says that both the Perishah of Yom Kippur and the Perishah of Parah Adumah are derived from the Milu'im. Tosfos points out that from Rashi's words it appears that the text of the Beraisa should omit the Perishah of Parah Adumah and say only that one Perishah is derived from the Milu'im (the Perishah of Yom Kippur). Accordingly, Rebbi Yochanan's opinion has support from a Tana in a Beraisa. Rebbi Yochanan sides with that Tana when he argues with Rebbi Yishmael.
(b) Tosfos gives a second answer. From the fact that Rebbi Yochanan earlier quotes Rebbi Yishmael as saying that the Torah requires Perishah for both Yom Kippur and Parah Adumah, it is evident that Rebbi Yochanan had a Beraisa (which is not quoted explicitly in the Gemara) which recorded the view of Rebbi Yishmael. That Beraisa apparently recorded the view of Rebbi Yishmael, who said that the Torah requires two Perishos, as well as the view of the Rabanan, who said that the Torah requires only one Perishah (that of Yom Kippur). Rebbi Yochanan here relies on the view of the Rabanan of that Beraisa when he argues with Rebbi Yishmael.
2) THE REASON FOR SEVEN DAYS OF "PERISHAH"
QUESTION: Reish Lakish derives the obligation to do Perishah from the Torah's account of Moshe Rabeinu's ascent to Har Sinai. Just as Moshe Rabeinu was required to do Perishah for six days before he approached the Shechinah, so, too, the Kohen Gadol is required to do Perishah before he enters the Kodesh ha'Kodashim on Yom Kippur.
From where, though, is the seventh day of Perishah derived? The Gemara answers that the Rabanan decreed that the Kohen Gadol separate for an additional day due to a concern that he might have had relations with his wife when she was a Nidah (that is, she discovered that she was a Nidah immediately after they separated), in which case he would be Tamei for seven days (as Rebbi Yehudah ben Beseirah says later on 6a).
If this is the source for the seventh day of Perishah, then why does the Gemara need to derive the first six days from the Perishah of Moshe Rabeinu? All seven days of Perishah are necessary due to the concern that the Kohen Gadol is Tamei as a result of being with his wife while she was a Nidah. The requirement for Perishah is a practical concern, to ensure that the Kohen Gadol is Tahor.
(a) The TOSFOS HA'ROSH answers that, indeed, according to Rebbi Yehudah ben Beseirah no other source is needed to teach the requirement of Perishah. (Those who derive it from Sinai or from the Milu'im follow the view of the Rabanan who argue with Rebbi Yehudah ben Beseirah and are not concerned that the Kohen Gadol will be Tamei as a result of relations with a Nidah. Consequently, perhaps they require only six days of Perishah.)
(b) The TOSFOS YESHANIM, and the Tosfos ha'Rosh in his second answer, explain that only six days of Perishah are required mid'Oraisa, as derived from Sinai. The Rabanan added a seventh day of Perishah because of a concern that the Kohen Gadol might have had relations with a Nidah.
Had there been no Torah-source for Perishah in the first place, the Rabanan would not have instituted seven days of Perishah merely because of a concern that the Kohen might have had relations with a Nidah.
(c) The RITVA says that if Perishah is necessary only because of the concern that the Kohen Gadol might have had relations with a Nidah, then it would suffice to require him merely to leave his home and his wife for seven days. Why must he be sequestered specifically in the Lishkas Parhedrin for seven days? It must be that Perishah is done to prepare him for an act of Kedushah -- entry into the Kodesh ha'Kodashim -- and not merely to prevent him from becoming Tamei. The source that the Kohen Gadol must be prepared for an act of Kedushah is derived from Moshe Rabeinu at Har Sinai.