We are in chapter 6 of Maseches Eduyos, mishnah 2. In this mishnah the Tannaim disagree about whether there is a special law governing a 'limb' that separates from a corpse, just as there is for a limb that separates from a living person.
The wording of the testimony:
"He'id Rabbi Yehoshua v'Rabbi Nechunyah ben Elinasan ish Kfar HaBavli al eiver min hameis shehu tamei" - these two Tannaim testified that a limb which separates from a corpse conveys tumah.
What is a 'limb' in this context:
When we speak of a 'limb' in this context, we mean a complete unit made up of three components:
The bone - a whole bone.
The flesh - the flesh surrounding that bone.
The sinews - the connective tissue that attaches the bone to the flesh.
The novelty of the law of a limb:
It should be noted at the outset that flesh and bone convey tumah even on their own, without the law of a limb: the flesh of a corpse conveys tumah in the amount of a kezayis, and a bone conveys tumah through contact and through carrying even in the amount of a bone the size of a barley grain - the size of a kernel of barley, a mere sliver of bone. But to convey tumah in a tent, that is not enough.
The novelty of the law of a limb is that even when none of these conditions is met - for example, the last, tiny joint of the little toe, whose bone is not the size of a barley grain and whose flesh is not the volume of a kezayis - nevertheless, if it qualifies as a 'limb', it conveys tumah not only through contact and through carrying, but even in a tent, meaning by being under one roof together with it. This is a law unique to a limb, and it is regarding this that they testified that it applies to a limb from a corpse as well.
The opinion of Rabbi Eliezer:
"Rabbi Eliezer omer: lo amru ela al eiver min hachai" - Rabbi Eliezer agrees that there is a special status for a complete limb, which conveys tumah even in a tent, but in his view this was said specifically about a limb from a living person - a limb severed from a living human being, such as a hand or foot that was cut off. Regarding a corpse, however, a limb has no independent status, and it is considered no more than the sum of its parts - the bone and the flesh in their own right.
The source of the law of a limb from a living person:
The source is the verse dealing with tumah transmitted by a corpse: "And whoever touches, in the open field, one slain by the sword, or one who died... shall be tamei for seven days." Now, a corpse conveys tumah no matter how the person died, so why does the verse specifically say "one slain by the sword"? Rather, it comes to teach that even a limb severed by a sword from a living person - a hand that was cut off or a leg that was cut off - conveys tumah just like a corpse itself.
Everyone therefore agrees that a limb from a living person conveys tumah, and even through an ohel (overhang), provided that it has the complete bone together with the flesh upon it. And this has a practical consequence: if a small amount of the flesh is missing, in a quantity that would grow back and heal on a living person, that deficiency does not remove from it the status of a limb; but if even a small part of the bone is missing, it is no longer a limb.
It emerges that the dispute is not about a limb from a living person, which by all opinions is a source of tumah, but about a limb from a corpse: does it too have a special status?
The Sages' kal vachomer:
"Amru lo: velo kal vachomer? Umah min hachai shehu tahor - ever haporesh mimenu tamei, hames shehu tamei - eino din sheyehei ever haporesh mimenu tamei" - Rabbi Yehoshua and Rabbi Nechunya answered Rabbi Eliezer: if you concede that a limb from a living person is a source of tumah, then all the more so a limb from a corpse. For the basic status of a living person is purity, and even so a limb that separates from him is tamei; a corpse, whose basic status is tumah, is it not logical that a limb that separates from it should be tamei?
Rabbi Eliezer's twofold reply:
From tradition: "Amar lahem: lo amru ela al ever min hachai" - this is the tradition I received, that the law was stated only regarding a limb that separates from a living person and not from a corpse, and with that the discussion is closed.
From a refutation: "Davar acher" - the kal vachomer itself can also be refuted. The basis of the refutation is the assumption that a corpse is more stringent than a living person; but that is not so, since there are respects in which a living person is more stringent than a corpse.
"Merubah tum'as hachaim mitum'as hameisim" - how so? "Shehachai oseh mishkav umoshav mitachtav letamei adam uletamei begadim, ve'al gabav madaf letamei ochlin umashkin - mah she'ein kein bames".
This refers to the tumah of midras: a living person who is a zav, a zavah, a niddah or a woman after childbirth conveys tumah by way of midras - when he places his weight on something designated to bear a person's weight, a bed or a seat. Even if ten mattresses were laid one on top of another and the zav lies on the topmost one, the bottom mattress becomes an av hatumah by the law of midras, and it has the power to make a person tamei and to make garments tamei.
And so too in the reverse direction: an object resting on top of the zav, the zavah, the niddah or the woman after childbirth becomes a 'madaf'. This is not an av hatumah but a rishon letumah, yet it still conveys tumah to foods and liquids. And even if ten mattresses are resting on top of the zav, the topmost of them is a rishon letumah even though it has no direct contact, and it makes foods and liquids tamei.
This mechanism of midras tumah does not apply to a corpse at all. It emerges that there is a way in which a living person conveys tumah and a corpse does not, and so there is no room for the kal vachomer, since we have found a respect in which a living person is more stringent than a corpse.
In summary: in this Mishnah Rabbi Yehoshua and Rabbi Nechunya testified that a limb from a corpse is tamei, and Rabbi Eliezer disagreed and held that the law of a limb was stated only regarding a limb from a living person, which is derived from the verse "one slain by the sword." The Sages sought to derive a kal vachomer from the living to the dead, and Rabbi Eliezer rejected them on two grounds: the tradition he had received, and the refutation that the tumah of the living is greater - for a living person creates a bed and a seat and a madaf, which is not the case with a corpse.
In the next Mishnah we turn to the limb from a living animal itself, which by all opinions conveys tumah through contact, through carrying, and through overshadowing, provided that it contains the entire bone, the flesh upon it, and the sinews that join them together.