Shevuos, Chapter 1, Mishnah 4. The Mishnah continues to deal with awareness of tumah, and now turns to the case of "v'al she'ein bah yediah lo bat'chilah v'lo basof" - a person who never knew at all that he was tamei, and during that time ate kodashim or entered the Beis HaMikdash, and in the end the matter is never revealed to him either.
Such a person can never bring a korban oleh v'yored, since there was no awareness at the outset, and even if the matter is later revealed to him he will not receive the atonement of an oleh v'yored. He does have a final atonement, as we learned in the previous Mishnah, through the goat offered outside on Yom Kippur. But what is his status in the meantime?
The opinion of Rabbi Yehudah:
"se'irei haregalim u'se'irei rashei chodashim mechaprin - divrei Rabbi Yehudah" - the goats brought in the Mussaf of the festivals and in the Mussaf of Rosh Chodesh are what atone for one who had no awareness either at the outset or at the end.
The opinion of Rabbi Shimon:
Rabbi Shimon disagrees and says: "se'irei haregalim mechaprin" - only the goats brought in the Mussaf of the festivals atone for one who had no awareness either at the outset or at the end, "aval lo se'irei rashei chodashim" - this is not the function of the goat brought in the Mussaf of Rosh Chodesh.
"Im kein, mah se'irei rashei chodashim mechaprin?" - for what, then, does the Mussaf goat of Rosh Chodesh atone? Rabbi Shimon answers: "al tahor she'achal es hatamei" - for a tahor person who ate kodashim that were tamei, which he is forbidden to eat. That is the atonement which that Mussaf provides.
The opinion of Rabbi Meir:
"Rabbi Meir omer: kol hase'irim kaparasan shavah - al tumas Mikdash v'kodashav" - all of these goats brought as sin offerings come for a single atonement, for every form of tumah of the Mikdash and its kodashim: for one who ate kodashim or entered the Mikdash while tamei, whether he had no awareness at the outset but had awareness at the end, or had no awareness either at the outset or at the end, and even for a tahor person who ate meat that was tamei. In his view there is no reason to distinguish between the goat offered outside on Yom Kippur and the goats of the festivals and of Rosh Chodesh; rather, all three atone for all three of these forms.
Nevertheless, even Rabbi Meir agrees to what was stated earlier, that the goat offered inside on Yom Kippur - the one brought in the Holy of Holies - suspends and protects the person who had awareness at the outset but has no awareness at the end.
An elaboration of Rabbi Shimon's opinion:
The Mishnah returns and spells out Rabbi Shimon's opinion, in order to follow it with the words of the Chachamim who disagree with him: "Hayah Rabbi Shimon omer" -
"se'irei rashei chodashim mechaperin al tahor she'achal ochel tamei" - the goats brought as musaf offerings on Rosh Chodesh atone for someone who ate consecrated food that was tamei.
"veshel regalim mechaperin al she'ein bo yediah lo bitchilah velo basof" - the goats of the festivals atone for someone who had no awareness at all.
"veshel Yom haKippurim mechaperin al she'ein bo yediah bitchilah aval yesh bo yediah basof" - the goat brought as the musaf of Yom Kippur atones for someone who had no awareness at the outset, but afterwards became aware of the matter.
The question of the Sages:
"amru lo: mahu sheyikrevu zeh bazeh?" - what is the law if the goat designated for the musaf offering of Yom Kippur was lost, and in its place one of the other goats that had been set aside for a festival was brought: may it be offered? "amar lahem: yikrevu" - Rabbi Shimon answered that it may be offered.
"amru lo: ho'il ve'ein kaparatan shavah, heichach kerevin zeh bazeh?" - after all, you disagreed with Rabbi Meir and held that the atonement is not the same in all three cases. According to Rabbi Meir, who holds that all the goats atone for all three categories of awareness of tumah, the matter is understandable; but according to your approach, how can one be offered in place of the other? How can the goat of Yom Kippur come in place of the goat of the festivals, when they do not atone for the same thing - the one of Yom Kippur atones for someone who had no awareness at the outset but became aware afterwards, while those of the festivals come for someone who had no awareness either at the outset or afterwards?
"amar lahem" - Rabbi Shimon answered them: even though they originally come for different purposes, they have a common element, in that they all come to atone for the tumah of the Mikdash and its consecrated items. The atonements are not identical, but they all atone for a matter connected to the tumah of a person who enters the Mikdash or eats consecrated food, and this common element is enough to permit offering one goat in place of the other - the goat of Yom Kippur for the sake of the festivals, and the reverse.