Ohalos, Chapter 17, Mishnah 5. The subject before us is a field that carries tumah even though we cannot point to any particular spot in it. We know that there is a kever somewhere in this field, but its exact location has been lost to us. Such a field is a beis hapras, and whoever walks across it must reckon with the possibility that at this very moment he is forming an ohel over a meis. The Mishnah now takes that difficulty one step further, and then moves on to a second topic: how much tamei earth is needed before we have a tumah at all.
A Kever Lost in the Field, and a House Built Above It
The Mishnah opens: "Kever besochah", there is a kever inside this field. Since we do not know where it lies, the entire area is treated with suspicion. If a person spreads an ohel anywhere over such a field, the whole ohel becomes tamei, for who can promise that it was not pitched directly above the grave?
The case grows more involved. "Ubanah bah bayis", the owner went ahead and erected a house on this ground, and not a single level either: "ve'aliyah al gavav", an upper chamber resting on top of it. We must therefore reckon with the possibility that the grave whose location nobody remembers lies underneath this structure, with a dwelling directly over it and a second story over that.
Step back and picture the building from the outside. The ground floor has a doorway in front. The upper story also has a doorway, reached by a ladder, by steps, by a porch, whatever the arrangement happens to be. The Mishnah's halachah turns entirely on how those two doorways line up.
The Mishnah rules: "Im haysa pischah", if the entranceway "shel aliyah", that of the upper chamber, is "mechuvan keneged", positioned exactly opposite, "pischah shel bayis", the entranceway of the dwelling below it, so that a person standing at the upper doorway and casting his eyes downward finds the lower doorway precisely beneath him, the ruling is "aliyah tehorah": the second story remains tahor.
What is the reasoning? Run through the possibilities. It may be that the forgotten grave sits under the main body of the lower level (in this Mishnah, "house" refers to that lower level). If so, the roof of that level covers the burial place, and such a covering locks the tumah in beneath it so that it cannot climb to the story above. Alternatively, the tumah may be caught underneath the lower entranceway. That too is sealed off, since the mashkof, the beam running across the top of the ground floor's entrance, constitutes an ohel stretched above the burial place and blocks the tumah's ascent. On either accounting, the aliyah escapes untouched.
"Ve'im lav, aliyah tmei'ah": when the two entrances do not correspond, one sitting off to one side and the other to the opposite side, the upper chamber contracts tumah as well. The worry runs like this. It is possible that the grave is situated beneath one of the walls of the lower dwelling, at a spot that falls exactly under where the upper story's entranceway is placed. A wall has nothing roofing it, so the tumah climbs upward unimpeded until it reaches the mashkof of the aliyah's entrance. That beam then serves as a roof spread above it, drawing the tumah in through the doorway and diffusing it throughout the upper chamber.
Afar Beis Hapras and Afar Chutz La'aretz
The Mishnah now turns to a different matter: earth that itself carries tumah. There is afar beis hapras, soil taken from such a field, and there is afar chutz la'aretz, soil brought from outside Eretz Yisroel, upon which Chazal likewise decreed tumah. Both are the same category of tumah, a tumah miderabbanan. And both tumas eretz ha'amim and tumas beis hapras are metamei bemaga, by touching the earth, and bemasa, by carrying it even without touching it.
The situation the Mishnah describes is "sheba beyarak", soil that shows up attached to produce. Perhaps a person picked greens that had been growing in a field with a lost grave in it. Or picture a guest who has traveled in from chutz la'aretz and brings a gift of fresh produce to his uncle, and the uncle looks it over and groans: clumps of dirt are still lodged around the roots, the very ground in which those vegetables were raised.
Rebbi Eliezer's ruling: "mitstarfin", the small amounts of soil clinging to the various stalks are combined with one another to reach the measure of "kechosam hamartzufin". A chosam means a seal, and a martzuf is a freight sack. The Bartenura describes a sizable leather sack in which merchandise was moved from place to place, fastened shut with a seal of clay. Sealing such a sack required a piece of earth or clay of a set size, and that piece is the measure under discussion here. In our case, then: every stalk carries only a small clod at its base, too little on its own, yet once the stalks are gathered into a single bundle the clods together reach the volume needed to seal a martzuf. Such is "divrei Rebbi Eliezer".
The Chachomim take issue: "VeChachomim omrim", they require "ad sheyehei bemakom echad", that in one single location there be a quantity matching "kechosam hamartzufin". For them it does not suffice to tally a bit from here with a bit from there. Unless a single spot holds an amount of earth equivalent to the seal of a freight sack, there is no tumah at all; soil collected out of different places is not joined.
Rebbi Yehuda's Proof
"Omar Rebbi Yehuda", Rebbi Yehuda cites an incident: "ma'aseh she'igros ba'os", letters would arrive "mimedinas hayam", from across the sea, addressed "livnei kohanim gedolim", to the children of the kohanim gedolim. It is easy to guess why so much mail reached them: the writers presumably hoped that these sons would lay their petitions before their fathers and obtain a berachah. Every such letter was fastened with a seal of clay, and clay is earth, and this earth had come from chutz la'aretz.
"Vehayah bahem kesa'ah okesatayim chosamos". All those seals put together came to a se'ah of clay, perhaps two. "Velo chashu lahem Chachomim", the Chachomim raised no objection "mishum tumah", on grounds of tumah. Consider who was handling this mail: kohanim, for whom tumah is a most serious matter. Why then the absence of concern? Because the shiur has to be found in one location. The clay on any individual letter never reaches the volume required for sealing a traveller's sack, and heaping all the seals into a mound until you have a se'ah or two changes nothing, since they are not situated together in one location. Rebbi Yehuda's story thus functions as a ra'ayah, a proof, supporting the Chachomim against Rebbi Eliezer: the earth must be present in a single spot and cannot be assembled from scattered crumbs.
Reviewing the Mishnah
Let us run through the Mishnah again. There is a field somewhere in which a grave went missing. Its soil transmits tumah through contact and through carrying, and as for the field itself, one who bends over it can become tamei, since its grave's location is unknown to us. Should someone pitch an ohel at any point in this field, that entire ohel is tamei: what assurance does he have that he did not set it up directly above the burial place?
Now consider a permanent structure rather than a tent: a dwelling, in fact one of two levels, a bayis with an aliyah on top of it. When the upper entrance and the lower entrance sit one directly over the other, the second story is tahor, since even should the grave happen to be under the lower entrance, the mashkof of that lower doorway acts as an ohel and confines the tumah below. When the two entrances are out of alignment, one shifted to this side and one to that side, the upper chamber becomes tamei too, since we must reckon that the doorway of the second level stands right above the tumah underneath. The tumah ascends, arrives at the mashkof of the upper entrance, and that beam functions as an ohel which brings the tumah into the upper room.
Finally we studied the earth of a beis hapras and the earth of eretz ha'amim. Both of them convey tumah bemaga and bemasa, and the required amount is whatever it takes to seal a traveller's sack, that special leather bag in which people carried their possessions on the road. And what of a man who shows up carrying a bundle of greens with dirt still clinging to the ends of the stalks? Rebbi Eliezer is machmir: the soil on one vegetable joins with the soil on the next, and once the total matches the measure of kechosam hamartzufin, this tumah applies. The Chachomim hold otherwise, demanding the full shiur in a single spot with no adding together. And Rebbi Yehuda supplied his proof: letters used to arrive mimedinas hayam for the children of the kohanim gedolim, closed with clay seals, and were you to total up all that clay you would have a great deal of it, a se'ah or two, yet nobody was ever troubled that the kohanim were being rendered tamei by them. Precisely for this reason: in no one place was there a sufficient quantity of tamei earth.