With the beginning of the seventeenth perek we begin learning about the Beit HaPras. The Mishnah first discusses the first type of beit hapras -- a field in which a grave has been ploughed through. The Mishnah teaches that for a distance of one-hundred amot from that grave, the soil from the field is an av ha'tumah such that it can make both vessels and people tameh. They would become tameh if one touched a the soil (maga) or carried a clod of earth or caused it to move (masa). The Bartenura explains this is because there is a concern that a small bone, the size of barley, may have been dragged along by the field for that distance. Since a bone that size is a source of tumah which can be transferred by maga and masa, the soil is considered a source of tumah in the same way.
The Gemara (Ketubot 28b) explains that a beit hapras is rabbinic in origin. The Tosfot there explains that that is referring to a case where the field is considered a reshut harabim (public domain). On a biblical level, a doubt regarding tumah in the reshut harabim is considered tahor. The Tosfot explain that this is ordinarily in the case where the location of the tumah is known and there is doubt in a particular instance whether the transfer occurred. In future however, the doubt can be avoided by taking more care. In our case however, the field itself is continually in doubt, so the Chachamim ruled stringently. We find however that if the field was in a reshut hayachid (private domain) the tumah would be considered biblical.
The Rashash however asks that we find in the case where a person walked on one of two paths in the public domain and it is not know which of the two paths had tumah, that we rule the individual is tahor. The Rashash answers that in that case, the paths will certainly be cleared and the tumah removed. This is because if one walked on both paths, then he would be deemed tameh. If however we ruled that one that walked on a beit hapras would be tahor, it would be unlikely that the field would ever be cleared.
The Mishnah Achrona on our Mishnah however finds the Tosfot difficult since he understands that the Mishnah in Taharot (4:5) does not differentiate between where the beit hapras is located.
The Tosfot in Moed Katan (5b s.v. menapeach) explains that beit hapras is rabbinc, since it is unlikely that the plough reached deep enough to hit the grave. Nevertheless, due to the seriousness of taharoth in general, the Chachamim applied a stringency by way of this gezeira. The explanation of the Tosfot is important because if there was a genuine doubt regarding the location of the bones, then it would be a considered a doubt regarding tumah in the private domain which is tameh on a biblical level. The Tosfot in Moed Katan however does not differentiate.
The Sidrei Taharot however notes that the Tosfot in Moed Katan does not fit the opinion in Ketubot. According to the opinion presented in Moed Katan, the location of the grave would certainly be tameh -- since it is more likely the bones there are intact. Furthermore, it should also be tameh in an ohel. According to the Tosfot in Ketubot however, even the location of the grave is treated like a beit hapras.
The Sidrei Taharot suggests that that the Tosfot in Ketubot has a similar understanding to the Ritva. The Ritva (ibid s.v. veha amar) explains that the reason why the tumah is rabbinic is because it can be assumed that the bones were significantly crush to pieces as a result of the plough. It would seems that the Ritva understands that we assume that the bones would certainly have been finely crushed such that they would be too small to be considered a source of tumah.1
1 The Ritva's explanation however would be enough to explain why beit hapras is rabbic even in a reshut hayachid. Furthermore the Sidrei Tahorot argues that the logic of the Ritva might be enough to assume the rest of the field is tahor since its original status was tahor, but not enough to convert the original grave from tameh to tahor. See the Sidrei Taharot inside for a full treatment of the Tosfot.
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