Shevuos, Chapter 1, Mishnah 7. This mishnah is a direct continuation of the previous one. At the end of the previous mishnah we learned that for all the transgressions other than tumah of the Mikdash and its consecrated items - those transgressions involving tumah, which implicate the Beis HaMikdash and the sacred foods - the goat that is sent away atones: for positive commandments and for negative commandments, for sins punishable by kareis and for sins punishable by death at the hands of the court.
Our mishnah opens by establishing that this law applies equally to everyone: Israelites, kohanim, the anointed kohen and the Kohen Gadol. All of them benefit from the atonement of the goat that is sent away for all the other transgressions.
Where, then, is the difference between Israelites, kohanim, the anointed kohen and the Kohen Gadol?
The only difference lies in the blood of the bull. Until now, in all the offerings we have discussed - on the festivals, on Yom Tov, on Rosh Chodesh and on Yom Kippur - we have been dealing with goats, the she-goats brought as a sin offering. But there is also a bull, an ox brought as a sin offering, and it is brought by the Kohen Gadol on behalf of himself and on behalf of the kohanim.
The blood of that bull is what provides the kohanim with atonement for "tumas Mikdash v'kodashav" - tumah of the Mikdash and its consecrated items:
A person in a state of tumah who entered the Beis HaMikdash.
A person in a state of tumah who ate consecrated food.
A person in a state of taharah who ate consecrated food that had become tamei.
All those laws of tumah of the Mikdash and its consecrated items, for which the atonement of Israel comes through the goats as we learned in the previous mishnayos - the atonement of the kohanim for them comes from the blood of the bull.
The view of Rabbi Shimon:
Rabbi Shimon disagrees on this point and says: "k'shem she'dam ha'sa'ir ha'na'aseh bi'Kodesh HaKodashim mechaper al Yisrael, kach dam ha'par mechaper al ha'kohanim" - just as the blood of the goat whose service is performed in the Holy of Holies atones for Israel, so too the blood of the bull atones for the kohanim. It should be emphasized that the goat's atonement for Israel regarding tumah of the Mikdash and its consecrated items takes place without any confession - there is no confession connected to that goat; rather, it is brought, and the blood itself provides the atonement. So too the blood of the bull atones for the kohanim for all cases of tumah of the Mikdash and its consecrated items.
However, over the bull there is a confession, just as there is a confession over the goat that is sent away. From here Rabbi Shimon continues: "k'shem she'viduyo shel sa'ir ha'mishtale'ach mechaper al Yisrael, kach viduyo shel par mechaper al ha'kohanim" - just as the confession over the goat that is sent away atones for Israel, so too the confession over the bull atones for the kohanim. The confession over the goat that is sent away atones for all of Israel for all the other transgressions, and corresponding to it, the confession over the bull atones for the kohanim for all the other transgressions.
What emerges from Rabbi Shimon's words: the goat that is sent away serves no function at all for the kohanim, since their atonement comes entirely from the bull - the blood of the bull atones for them for tumah of the Mikdash and its consecrated items, and the confession over the bull atones for them for all the other transgressions. It follows that the goat that is sent away is brought for the Israelites alone.