Ohalos, Chapter 2, Mishnah 7. This chapter is occupied with the various measures of tumah that comes from a meis: how much of a substance is needed for it to be metamei by touch (maga) or by carrying (massa), and how much is needed for it to spread tumah to everything under one roof (ohel). Our mishnah takes up three separate cases in which the required amount exists, but not in one unbroken piece, and it asks whether the pieces still join together to form a shiur.
A bone the size of a barley grain that was split
The mishnah begins: "etzem k'se'ora shenechlak lishnayim," a bone from a meis that measures a barley grain and was divided into two. This is precisely the smallest measure of bone that carries tumah through maga and massa. Now that it lies in two fragments, neither fragment on its own reaches the shiur, yet placed together the two of them do amount to a full barley grain.
On this the mishnah records: "Rebbe Akiva metamei," while "Rebbe Yochanan ben Nuri metaher." Rebbe Akiva's position is that the two halves join to make up the measure, so that a person who picks up both fragments and carries them, or who makes contact with the pair of them at one moment, has become tamei. Rebbe Yochanan ben Nuri holds the opposite: the barley-grain quantity must be found in one continuous piece of bone. On his view, contact with two half-measures together, or carrying them together, leaves the person unaffected and tahor.
It is worth appreciating just how far apart these two Tannaim stand. Rebbe Akiva, in his view, does not even require that the two fragments originate in the same body; as taught in the preceding mishnah, he joins together bone from two different meisim. Why, then, does our mishnah present him arguing about a single bone that broke in half? Precisely in order to sharpen the position of Rebbe Yochanan ben Nuri, who takes the opposite extreme: even though the bone came from one meis and was one bone until a moment ago, once it is not a single piece it does not carry tumah.
The Meleches Shlomo asks an obvious question here. In the mishnah before this one there were Sages who refused to combine bone taken from two different meisim; how would they rule in our case? The meforshim explain that in our case they too would declare it tamei, since all of this bone comes from one body, and their reservation was limited to material drawn from two separate people. Rebbe Yochanan ben Nuri, though, is not prepared to grant even that much: whether it came from one body or from two makes no difference to him, for a shiur can never be built up out of separate fragments.
He states his principle in his own words: "lo omru etzem k'se'ora ela etzem k'se'ora." When the Chachomim fixed a barley grain as the measure for tumah, they meant one whole bone of that size, not two pieces that add up to it. And a single bone that has broken counts, for this purpose, as two.
A quarter kav of bones ground to powder
The mishnah continues with a second case: "rova atzamos shenidagdeku," a quarter of a kav of bones that were pulverized. The word "nidagdeku" carries the sense of the phrase "veshachakta mimenu hadek," grinding something down fine; these bones have been reduced to genuinely tiny fragments. And the mishnah specifies the crucial detail: "ve'ein bechol echad ve'echad etzem k'se'ora," not one of the fragments amounts to a barley grain.
"Rebbe Shimon metaher." In Rebbe Shimon's opinion this entire quantity is tahor and transmits no tumah whatsoever, neither by maga, nor by massa, nor by ohel. "Vachachomim metamin": the Chachomim hold that it is tamei in all three respects, since the required quantity is still there in front of us, crumbs and all.
To feel the dispute, keep the two measures in mind. A barley grain of bone is what is needed for tumah of maga and massa. A quarter kav of bones is what is needed for tumah of ohel. Here we have a quarter kav, which is the ohel measure, but it has been broken into particles so small that not one of them holds a barley grain. Rebbe Shimon says that once the material has been reduced below the smallest meaningful unit, nothing remains that can be metamei. The Chachomim answer that the quarter kav is intact as a quantity, and therefore the tumah of ohel applies, along with maga and massa.
A limb severed from a living person
The third case concerns a limb taken from a living person, eiver min hachai, which carries tumah when it is complete with its flesh, sinews and bone. The mishnah teaches: an eiver that was separated from a live person and then "nechlak lishnayim," split into two, is tahor. Nor does it help to press the two halves back together; the limb has lost its status. "Rebbe Yosi metamei," Rebbe Yosi holds that even now it conveys tumah.
But even Rebbe Yosi has a limit: "umodeh Rebbe Yosi shenital chetzyo shetahor." If the limb never left the living body as a whole limb, but rather half of it came away first and afterward the second half came away, Rebbe Yosi agrees that it is tahor. His ruling applies only where a complete eiver once existed and was afterward divided; where the limb never held its shiur for even a moment, there is nothing to be metamei.
The halachah, however, does not follow Rebbe Yosi. The Rambam paskens that even if the limb was removed intact, once it has been split in two it is tahor.
A closing thought
These are demanding halachos to sit with, and one can wonder what we take away from measures of bone and barley grains. I think of something my father, zichrono livrachah, once told me about a very choshuv rov, whose name I no longer recall, who would toil over hilchos Pesach year after year. We are meant to learn hilchos Pesach from Purim until Pesach, and he would learn them every single year as though he had never seen them before, every se'if in Shulchan Aruch. He remarked that in the years he learned the halachos thoroughly, the shailos simply did not come up; in the years he learned less, complicated shailos arrived, and he found himself searching and searching to determine the din. As though the Torah asks only that we learn it, and having learned it, we have discharged our duty.
The same applies to us. We sit and learn these halachos of tumah and taharah, and the learning itself is already a brachah. May the learning be all that is asked of us, and may everyone stay well, whole and gezunt in guf and in neshamah alike.