Ohalos, Chapter 18, Mishnah 5. This chapter is devoted to the beis hapras, a field that carries a rabbinic tumah because of the possibility that fragments of bone are scattered through its earth. Our mishnah takes up the practical question: once such a field exists, is there any way to restore it to a state of taharah? The mishnah opens with exactly that question, "Keitzad me'ta'arin beis hapras," how does one purify a beis hapras?
Which Beis Hapras Are We Discussing?
To follow the mishnah we have to return to the first of the categories of beis hapras: a field in which someone plowed over a kever. The grave itself is no longer intact; the plow broke it up and may have dragged slivers of bone along with the soil. That is the whole concern here, small fragments carried by the blade. This is a crucial point to keep in mind before we begin, and we will return to it at the end of the lesson.
Removing Three Tefachim or Adding Three Tefachim
The mishnah's first solution: "Notlin mimenu shelosha tefachim," one removes the top three tefachim of soil from the field. The logic rests on the reach of the plow. The machresha has a blade that cuts into the ground and turns the earth over, which is what allows seeds to be set into the soil, and that blade descends only three tefachim. If the entire worry is that the plowman dragged bones through the ground, he could not have carried them any deeper than his blade reached. Strip off those three tefachim and whatever the plow disturbed is gone.
There is a second option: "O nosnin al gavav shelosha tefachim," or one places three tefachim of tahor earth on top of the field. Now the layer that might contain fragments lies three tefachim beneath the surface. Even if someone comes and plows this field, his blade will pass entirely through the clean soil brought from elsewhere and will never reach down to the bones.
A Compromise: Half and Half
What if someone starts with one method and finishes with the other? The mishnah permits this as well. "Im natal me'chatzi echad shelosha tefachim," if he stripped three tefachim of soil off half the area, and then decided he had dug enough and brought in a wagonload of clean earth instead, "ve'nasan al gabei chatzi ha'acher shelosha tefachim," spreading three tefachim of tahor soil over the remaining half, the ruling is tahor. Every part of the ground has been treated by one of the two acceptable procedures, and the field in its entirety is therefore clean.
Rebbi Shimon: A Tefach and a Half
Rebbi Shimon considers a depth of three tefachim excessive. The mishnah records his position: "Afilu natal mimenu tefach u'mechtza, o nasan alav tefach u'mechtza mimakom acher," tahor. Whether the owner scraped away one and a half tefachim from the top of the field, or laid down soil of that same depth brought in from elsewhere, the field is clean. In Rebbi Shimon's view a plow blade never reaches below tefach u'mechtza, so that thin layer is the whole extent of what requires our attention.
The Mishneh Acharonah asks the obvious question: how can Tannaim disagree over something one could simply measure? Take a plow and check how far down the blade cuts. His answer is that neither opinion has the facts wrong. A plain, everyday machresha indeed sank no deeper than tefach u'mechtza, but heavier plows also existed, and those reached a full three tefachim. The dispute is really about the scope of our concern. Since the tumah of beis hapras was instituted by Chazal, Rebbi Shimon holds that they obligated us to reckon with the common plow alone, and that shallower depth is therefore adequate. The Tanna Kamma holds that the larger implement must be taken into account as well, which is why he requires three tefachim.
Paving the Field With Stones
The mishnah continues with another approach: "Ha'rotzeif beis hapras," one who paves over a beis hapras. How is the paving done? "Ba'avanim she'ein yachol lehasi'an," with stones that cannot be budged. This is not a matter of scattering pebbles across the surface; the stones are set into the ground so solidly that they will not shift under a person's foot. In that case the mishnah rules: tahor. Since the stones do not move when one walks on them, there is no concern about a bone lying underneath. Had the stones wobbled as one stepped on them, we would have to worry that the movement of the stone is causing a bone below to move as well, and the field would remain tamei.
Rebbi Shimon: Digging Out the Stones
Rebbi Shimon offers yet another remedy: "Af ha'oizeik beis hapras." To be oizeik means to dig through the field and clear out all of its stones. If a person actually did that work in a beis hapras, the field is tahor. The reasoning is straightforward: a man who dug out every stone in the field would surely have come upon any bone fragments in the course of the digging, and we assume they were found and removed.
The Limits of These Remedies
It bears emphasizing once more which field we have been discussing throughout: a field that was plowed over a kever. We are not speaking about a field in which a kever is known to be standing. Where we know there is a grave present but cannot locate it, none of these remedies are of any use; the kever itself must be found. All of the above applies only where the sole concern is scattered fragments of bone.
Let us gather together the remedies the mishnah has offered. One may excavate three tefachim of the upper soil; one may cover the field with three tefachim of clean earth; or one may treat half the area by digging and the other half by covering. Rebbi Shimon reduces the required measure to tefach u'mechtza, whether by removal or by addition. A separate route is paving the ground with stones, provided they are set firmly enough that a footstep cannot rock them and thereby disturb a bone lying beneath. And Rebbi Shimon adds one more: a person who dug out every stone in the field and genuinely completed the task has rendered it tahor, since any splinters of bone would inevitably have surfaced while he worked.