Shevuos, Chapter 1, Mishnah 5. This mishnah is a continuation of the previous one, and to a certain extent stands in contrast to it. In the previous mishnah we learned the opinion of Rabbi Shimon, according to which the goats of Rosh Chodesh atone for a pure person who ate impure sacrificial food; the goats of the Musaf offerings of the festivals, of Yom Tov, atone for one who had no awareness either at the beginning or at the end; while the Musaf goat of Yom Kippur atones for one who had no awareness at the beginning but did have awareness at the end.
The question that was raised before him was whether an animal left over from one of these may be offered for a different Musaf, and he answered that it may. The reason, in his words, is that they all share a common denominator connected to the matters of the impurity of the Temple and its sacred items - one who ate sacrificial food in a state of impurity, or one who entered the Temple while impure.
In our mishnah a different version of Rabbi Shimon's opinion is taught, according to which the various goats build one upon the other, as will be explained.
The opinion of Rabbi Shimon ben Yehudah in the name of Rabbi Shimon:
"Rabbi Shimon ben Yehudah omer mishmo" - Rabbi Shimon ben Yehudah says these words in the name of Rabbi Shimon, that is, Rabbi Shimon bar Yochai. According to his approach there are three levels of atonement, each one building upon the one before it:
"Se'irei roshei chodashim mechaprin al hatahor she'achal es hatamei" - the Musaf goats brought on Rosh Chodesh atone for a pure person who ate something impure. This is the very same law that Rabbi Shimon stated in the previous mishnah.
"Mosif aleihem shel regalim, shemechaprin al hatahor she'achal es hatamei ve'al she'ein bo yediah lo bitchilah velo basof" - the goats of the festivals do not come for a different atonement, but for an additional one: they atone for a pure person who ate something impure, and in addition for one who has no awareness either at the beginning or at the end.
"Mosif aleihen shel Yom HaKippurim, shehen mechaprin al hatahor she'achal es hatamei, ve'al she'ein bo yediah lo bitchilah velo basof, ve'al she'ein bo yediah bitchilah aval yeish bo yediah basof" - to the Musaf goat of Yom Kippur a third layer is added: it atones for a pure person who ate something impure, just as the other two atone; for one who has no awareness either at the beginning or at the end, just as the goats of the festivals atone; and in addition for one who has no awareness at the beginning but does have awareness at the end, and this is the detail unique to the goat of Yom Kippur.
The question of the Sages:
"Amru lo" - the Sages asked Rabbi Shimon regarding this version as well: may an animal left over from one of them be offered for a different Musaf? "Amar lahen: hen" - and he answered that it may.
"Amru lo" - and they raised a difficulty with this: if so, it is indeed logical that the goat of Yom Kippur, which includes within it all three levels of atonement, may be offered on Rosh Chodesh, since its atonement includes the atonement of Rosh Chodesh. But a goat of Rosh Chodesh, which atones for only one thing, how can it be offered on Yom Kippur and effect an atonement that does not belong to it?
Rabbi Shimon's answer is identical to his answer in the previous mishnah: they still all share a common denominator, in that they all atone for a matter connected to impurity involving sacred items - whether the person is impure or the sacred food is impure.
In summary: This mishnah teaches an additional version of Rabbi Shimon's opinion, cited by Rabbi Shimon ben Yehudah in his name, according to which the atonement of the goats is cumulative: the goats of Rosh Chodesh atone for a pure person who ate something impure; the goats of the festivals add to that one who has no awareness either at the beginning or at the end; and the goat of Yom Kippur adds also one who has no awareness at the beginning but does have awareness at the end. Even so, his conclusion remains that one may be offered in place of another, because of the common denominator shared by them all - atonement for the impurity of the Temple and its sacred items.