בבא בתרא נה:
"טומאת קבר"
The Gemara says the מחלוקת between ר' אסי and רבין if ר' יוחנן said a מצר וחצב (a boundary and a chatzav plant) are considered partitions and divide a field even לענין טומאה, is relevant to the following shaila in שיטת ר' אליעזר: The Mishnah states: if one walked through a בקעה during rainy season, a time when the fields are planted and people avoid walking there, we therefore consider this valley רשות היחיד. If we know for certain that there is טומאה (a grave) in one of the fields and one walked through the valley but is uncertain if he entered that specific field with the קבר, ר' אליעזר says even though ספק טומאה ברשות היחיד טמא, here he is טהור, because this is a case of "ספק ביאה". The רשב"ם explains that this case is a ספק ספיקא: 1) Maybe he never entered the field with the קבר. 2) Even if he did, he might not have touched the טומאה. So, according to רבין who says ר"י holds a מצר divides fields even regarding טומאה, in the above case he will be טהור, because the fields are separate and therefore there is a ספק ספיקא. But according to ר' אסי, that a מצר doesn’t divide regarding טומאה, even if the fields had a מצר between them, they would all be considered one field and consequently, the ספק ספיקא would fall away andר' אליעזר would hold he is טמא.
The Gemara [פסחים דף י] brings a מחלוקת between רבי and רשב"ג in a case where a קבר was certainly in a field, but now its exact location is lost to us. If someone traverses this field, he is טמא because we are concerned, he might have been מאהיל over the lost קבר. If, however, a קבר is found in this field, רבי holds that we may assume this is the original קבר that was lost to us and one may now travel across this field and as long as he avoids this grave he will remain טהור. רשב"ג holds we may not assume this found קבר is the lost one and therefore the entire remainder of the field must be inspected. The [שמעתתא ז' פרק יז] שב שמעתתא brings a תשובת מבי"ט where a man left town on a journey and was never heard from again. His wife remained an עגונה. Soon after, a goy came to town and related that he had seen a dead man (unrecognizable) on the side of the same road that our missing husband had traveled on! The מבי"ט paskened that they may assume this corpse is the missing husband. The שב יעקב says the above Gemara is a proof to the psak of the מבי"ט. Just like we pasken like רבי that we may assume the found kever is the same one that was lost, so too, we may assume that the corpse that was found was the husband who was lost.
The שמעתתא does not like this comparison. In the case of the field, normally we would never entertain any possible Tumah. It is only because of one kever that it has now become טמא. So, when we find a קבר we have now found the "סיבת הספק" and when that’s removed the field reverts back to its original status which was that of any other field, טהור, unless proven otherwise. In our case, the women has a חזקת אשת איש and finding this corpse is not enough to take her out of her existing חזקת אשת איש. Secondly, by the קבר the exact item missing is the one that is found. By the lost traveler, a live person was lost and a dead body was found. ע"ש.