Chullin, Chapter 6, Mishnah 3. This chapter is devoted to kisuy hadam, the mitzvah of covering the blood of a slaughtered chayah or bird. The previous mishnah established the governing principle: the obligation to cover the blood exists only where the shechitah itself was valid. Our mishnah now applies that principle to a case where the validity of the shechitah is precisely what is in question, because the one holding the knife was a person whose actions we cannot rely upon.
Three people whose shechitah is suspect
The mishnah opens: "Cheresh, shoteh, vekatan sheshachtu ve'acherim ro'in otam, chayav lechasot." A cheresh, a deaf-mute; a shoteh, a person who lacks the mental capacity to be responsible for his actions; and a katan, a minor. These three are grouped together throughout the Mishnah because none of them can be presumed to perform a halachic act correctly. Left on their own, we have no confidence that the shechitah will be carried out according to its laws.
The case here, however, is one where acherim ro'in otam, others stood by and watched the shechitah from beginning to end, and observed that every requirement was fulfilled. Since the shechitah was in fact done properly, the meat is permitted and the mitzvah of kisuy hadam is fully in force. The blood must be covered.
But who covers it? Not the one who slaughtered. A cheresh, a shoteh and a katan are exempt from the mitzvot, and therefore they cannot discharge this obligation. The duty falls upon the observers. Those who watched the shechitah and can testify that it was valid are the ones who must perform the covering.
When no one was watching
The mishnah continues: "Bein levein atzman, patur milechasot." If these same people slaughtered privately, with no one observing them, whoever later comes upon the uncovered blood bears no obligation to cover it. The reasoning is the presumption we began with: since these three do not possess the ability to perform shechitah reliably, we assume the shechitah was invalid, and invalid shechitah carries no mitzvah of kisuy hadam. Only when someone actually watched and confirmed that the act was done correctly do we treat it as a valid shechitah.
The same question regarding oto ve'et bno
"Vechen le'inyan oto ve'et bno." The identical reasoning is now transferred to the issur taught in the previous perek, which forbids the killing of a parent beast and its young within one calendar day. "Sheshachtu ve'acherim ro'in otam, asur lishchot acharehen." Where one of these three carried out a shechitah while bystanders confirmed that each requirement had been met, the second member of that pair, be it the mother or her young, may not be slaughtered until the day has ended. A halachically effective shechitah has occurred, and with it the issur of oto ve'et bno has taken hold.
"Bein levein atzman", where the shechitah was done privately with no observers, there is a machloket.
Rabbi Meir
"Rabbi Meir matir lishchot acharehen." Rabbi Meir permits slaughtering the mother or the offspring later that same day. He holds consistently to the presumption stated above: when no one was watching, we assume the first shechitah was not done correctly. If the first animal was never validly slaughtered, the prohibition of oto ve'et bno was never activated, and there is no reason to hesitate before slaughtering the second animal of the pair.
The Chachamim
"Vachachamim osrin." The Chachamim forbid slaughtering the mother or offspring that day. In their view we cannot simply write off the first shechitah as invalid. There is a chashash, a real concern, that these people did in fact perform a proper shechitah. Since oto ve'et bno is a Torah prohibition, we must act stringently in the face of that doubt.
Consequently, the Chachamim would arrive at a parallel conclusion in the opening case of kisuy hadam. Where a bird or a chayah was shechted by one of these three with nobody present to verify the act, the doubt itself would generate an obligation to cover, since safek d'oraita lechumra: a Torah requirement in doubt is resolved strictly. The exemption recorded in the mishnah's first clause thus reflects the view of Rabbi Meir, whose starting assumption is that the shechitah never came out right.
Where even the Chachamim concede
The mishnah closes with a point of agreement: "Umodim she'im shachat", both sides hold that one who went ahead and slaughtered the second member of the pair during that day "she'eino sofeg et ha'arba'im", he is not given the forty malkut. The restriction the Chachamim imposed was a precaution born of doubt, not a verdict of certainty. Since it remains fully possible that the original act of shechitah accomplished nothing halachically, in which case killing the second animal transgressed no prohibition whatsoever, lashes cannot be administered.