Bechoros chapter 3, Mishnah 4, three mishnayos to the end of the chapter, and we continue to deal with the restrictions placed on shearing the wool of a bechor. From the Torah, once the sheep is slaughtered - whether it was blemished or unblemished and offered as a sacrifice - the wool on its skin becomes permitted. The question is what the law is regarding hair that was detached from the animal before its slaughter. From the Torah, it certainly should have been permitted, since it fell off on its own; but the Sages placed restrictions on this, so that they would not keep the bechoros around, lest they come to shear them or work with them, or lest some other mishap occur to them. Because of this, the Sages did not want to give any incentive to the owner or the kohen (and usually it is a kohen) to keep the animal instead of slaughtering it and eating it.
The question, then, is what the law is regarding the wool that was detached before the slaughter - after the death of the animal. Everyone agrees that if it was slaughtered properly, the wool on its skin is permitted; and everyone agrees that if it was not slaughtered according to the law of a blemished animal but died on its own, it requires burial, and this includes the wool attached to it. The doubt is only regarding the detached hair, and our Mishnah comes to answer this.
The structure of the Mishnah - a dispute over the parameters of the dispute:
This Mishnah is structured in a way that is sometimes hard to follow, since it is a dispute over what the dispute actually was about. The disputants are Akavya ben Mahalalel and the Sages, but their position is the subject of a dispute: the first opinion is Rabbi Yehuda's approach in defining the subject of their dispute, and the second opinion is Rabbi Yosi's approach.
The approach of Rabbi Yehuda:
"Se'ar bechor baal mum shenashar, vehinicho bachalon, ve'achar kach shechato" - The wool of a blemished bechor that was meant to be slaughtered and eaten by the kohen, and hair had fallen from it beforehand (for instance, the sheep walked past a thornbush and some wool was plucked from it). The kohen placed the wool on the windowsill for storage, and later properly slaughtered the animal. What is the status of that wool?
"Akavya ben Mahalalel matir" - The wool on the animal is certainly permitted now; and even the wool that was detached earlier, which the Sages forbade as long as the animal was alive so that they would not keep it longer than necessary, once it was slaughtered the reason is nullified, and therefore the wool in the window is permitted.
"Vachachamim osrin" - The wool that was detached before the animal's death remains forbidden. And when the Sages said it is forbidden, their intention was to forbid it forever, so that a person would have no incentive whatsoever to keep the animal around.
This is Rabbi Yehuda's approach, and this is how the halachah was decided - in accordance with the opinion of the Sages as he understood it.
The approach of Rabbi Yosi:
"Amar Rabbi Yosi: Lo bazeh hitir Akavya" - This is not the scenario in which Akavya was lenient, because in such a case even the Sages are lenient. The dispute revolves around a different case: "Ela bese'ar bechor baal mum shenashar, vehinicho bachalon, ve'achar kach mes" - A blemished bechor ready for slaughter from which hair fell, and he placed it in the window, and afterwards the animal died on its own (it was injured, died of old age, and the like) and was not slaughtered properly.
In this case, everyone agrees that the wool attached to the body is forbidden and requires burial, and the doubt is only regarding the wool on the windowsill. For according to Rabbi Yosi, had the animal been slaughtered properly, the hair in the window would have been permitted even according to the Sages.
"Bazeh Akavya ben Mahalalel matir" - Even though it is no longer possible to slaughter it, ultimately the animal is no longer alive, and there is no longer any reason to forbid it; the permission takes effect, and the wool in the window may be used.
"Vachachamim osrin" - Specifically in the case where the animal was not slaughtered, the wool in the window is forbidden.
Thus, according to Rabbi Yosi, the Sages are lenient regarding an animal that was slaughtered: just as the hair attached to its body is permitted, so too the hair that already separated from it and was placed in the window is permitted. Rabbi Yosi's approach is therefore much more lenient. In any case, the halachah follows Rabbi Yehudah's understanding of the Sages, who are stringent and leave the detached hair forbidden forever.
Dangling wool - the intermediate case:
The final section of the Mishnah deals with an intermediate case: hair that detached from its root on the back of the animal but had not yet fallen to the ground, and instead became tangled in the animal's wool. When the animal is sheared permissibly after slaughtering, the hair that detached before death, which the Sages forbade, becomes mixed into that same fleece. Is that hair also forbidden?
"Hatzemer hameduldal babechor, shehu nir'eh min hagizah" - dangling wool on a bechor, which appears as part of the fleece itself and one cannot distinguish between what is attached and what is detached, since it blends into it and creates a uniform surface of wool - is permitted. In this case, the Sages were not stringent: as long as it is not noticeable, the wool remains in the owner's possession.
"Veshe'eino niker min hagizah" - wool that is noticeable to the eye, dangling from the edge of the fleece, where it is clear that it was not attached to the animal at the time of slaughter but was detached beforehand - is forbidden forever, and must be removed from the fleece.