1) THE SOURCE FOR "HAZA'AH" DURING THE PERIOD OF "PERISHAH"
QUESTION: The Gemara says that just as Haza'ah with blood was performed on Aharon ha'Kohen for seven days in the Mishkan at the time of the Milu'im, so, too, Haza'ah with water is performed on the Kohen Gadol before Yom Kippur, and on the Kohen who prepares the Parah Adumah, for all seven days of their Perishah. Even though the original Haza'ah was done with blood, "water comes and takes the place of blood" ("Nichnas Mayim Tachas Dam").
Why is the Haza'ah of water comparable to the Haza'ah of blood, such that one can be learned from the other? Water is used to be Metaher the Kohen from Tum'as Mes, a process entirely unrelated to the Haza'ah of blood.
(a) The TOSFOS HA'ROSH explains that although the two Haza'os are not similar to each other, the Gemara derives merely the general requirement to perform a Haza'ah during Perishah from the Haza'ah of the Milu'im. It does not derive the specific type of Haza'ah from there.
(b) TOSFOS (DH Nichnesu) suggests that according to this Tana, the source from the Milu'im is only an Asmachta. The simple meaning of the verse is that the procedures which are done on the first day of the Milu'im must also be done all seven days of the Milu'im; the verse does not refer to Yom Kippur or Parah Adumah. The verse is merely an Asmachta upon which the Rabanan relied as a support for the requirement of Perishah for Yom Kippur and Parah Adumah.
Tosfos proves this from the Gemara later (8a) which gives a different reason for the Haza'ah during Perishah. The Gemara there says that each day of the Perishah might be the third or seventh day after the Kohen touched a corpse. Accordingly, the reason for Haza'ah is unrelated to the verse of the Milu'im. It must be that the verse is only an Asmachta. (It appears that Tosfos says this only according to the Tana who authored the Beraisa that mentions only one Perishah (see Insights to Yoma 3:1). According to the Tana who mentions two Perishos, the requirement of Perishah is mid'Oraisa, as is clear from the discussion at the beginning of the chapter about how to explain the verse.)
2) DERIVING "PERISHAH" FOR YOM KIPPUR FROM MOSHE RABEINU'S "PERISHAH" AT SINAI
QUESTION: The Gemara cites support from a Beraisa for Reish Lakish's opinion that the Kohen Gadol's requirement of Perishah before Yom Kippur is derived from Sinai. Rebbi Yosi ha'Glili in the Beraisa says that a cloud covered Moshe Rabeinu for six days, and only on the seventh day did Moshe Rabeinu ascend the mountain to the place of the Shechinah (Machaneh Shechinah).
How does Rebbi Yosi ha'Glili's statement in the Beraisa about the Perishah of Moshe Rabeinu at Sinai teach the requirement for Perishah of Yom Kippur?
(a) TOSFOS (DH Tanya) answers that the Gemara later (4b) quotes Rebbi Elazar, a student of Rebbi Yochanan, who agrees with Rebbi Akiva's opinion that the six-day Perishah at Sinai was done prior to Matan Torah. It is likely, therefore, that Rebbi Elazar's teacher, Rebbi Yochanan, also followed the view of Rebbi Akiva (as the Gemara says in Shekalim 7b).
Why, though, did Rebbi Yochanan follow the view of Rebbi Akiva if Rebbi Akiva's opinion is less consistent with the verses than Rebbi Yosi ha'Glili's opinion? Moreover, two other Tana'im in the Beraisa disagree with Rebbi Akiva and maintain that Moshe Rabeinu's Perishah followed Matan Torah, and thus Rebbi Yochanan should have agreed with the majority opinion and not with Rebbi Akiva. It must be that Rebbi Yochanan sided with Rebbi Akiva only because he had a tradition that the Milu'im is the source for Perishah. If Moshe Rabeinu practiced Perishah before he ascended Har Sinai, then it is much more logical to derive Perishah of the Kohen Gadol from the Perishah at Sinai, as Reish Lakish says, than to derive it from the Milu'im. Moshe Rabeinu's Perishah at Har Sinai was done in preparation for his entry into the Machaneh Shechinah, similar to the purpose for which the Perishah of the Kohen Gadol before Yom Kippur is done.
Accordingly, Rebbi Yosi ha'Glili's opinion serves as support for Reish Lakish. (This appears to be the intention of Tosfos. See the GEVURAS ARI who discusses Tosfos at length.)
(b) The GEVURAS ARI and SI'ACH YITZCHAK explain that it is clear from the Beraisa that Rebbi Yosi ha'Glili maintains that the Perishah at Sinai is the source for the Kohen Gadol's Perishah before Yom Kippur. Rebbi Nasan in the Beraisa argues with Rebbi Yosi ha'Glili and says that the Perishah of Moshe Rabeinu was done to "digest the remains of food in his intestines." Since both Rebbi Nasan and Rebbi Yosi ha'Glili agree that Moshe Rabeinu's Perishah was done after Matan Torah as Rashi explains, in what respect do Rebbi Nasan and Rebbi Yosi ha'Glili disagree?
It must be that they disagree about whether the Kohen Gadol needs Perishah every year before Yom Kippur. Rebbi Nasan maintains that only Moshe Rabeinu needed Perishah, since he would have no food for forty days while in Shamayim and would live like an angel (as Rashi explains). Rebbi Yosi asserts that Moshe Rabeinu's Perishah was a preparatory act for entry into the Machaneh Shechinah, and thus the Kohen Gadol similarly needs to do Perishah before every Yom Kippur.