Having discussed the nature of tumah associated with a zav and the means of its transfer, the third perek discusses various situations shared by a zav and another person and whether that person becomes tameh. The first of these cases is where they are both on a small boat or raft. The Mishnah rules that even if their clothes are not in contact, "they" are tameh midras. What does this mean?
To explain, unlike many source of tumah where the tumah is transferred via contact alone, a zav can transfer tumah by other means as well. It can be transferred by heiset (if the zav is moved or causes something else to move) or masa (he is carried). Furthermore, the zav can effect tumat midras. This means that if the zav stands, lies, sits, leans or is raised by (nitla) an object that it suited for sitting or lying on, then it becomes an av ha'tumah (source of tumah). Consequently, when our Mishnah states that that they become tameh midras, one would expect it to be referring to the clothing of the zav. We shall try to understand why.
The Bartenura explains that since the boat is small, sometimes the zav may weigh one side down, causing the tahor to rise thereby making him and his clothes tameh through heiset.
The Mishnah Achrona raises a number of difficulties. Firstly, he notes that the Mishnah used the term midras and not heiset. He explains that the difference between the two is that, as we have explained, an object affected by tumat midras becomes an av hatumah while one that becomes tamah by heiset is a rishon le'tumah.1
Furthermore, he argues that the Bartenura should have explained the Mishnah like the Rosh, that sometimes the opposite may occur where the zav is raised by the weight of the tahor, which would be equated to one of the cases of tumat midras (nitla).
The Mishnah Achrona therefore explains that the Mishnah here can only be referring to the clothing since that is the only thing that can be affected by tumat midras. Furthermore, he cites the Rambam that tumat midras in this case is not a certainty, but rather a concern; given the movement in the small raft, perhaps the zav was momentarily resting on the clothes of the tahor.
Why did the Rambam not explain like the Rosh? He answers citing a later Mishnah (4:7) that teaches as follows. If a zav was seated on a bed with each of its legs resting on a separate tallit then each of them are tameh. If however the zav was on an animal that was standing with each leg on another tallit then they are tahor. The reason is, that the animal is able to stand on three legs. The fourth leg (which could be any of them) is only assisting and is disregarded. We find therefore that if it is only contributing, we discount it. Consequently, in this case, since it is primarily the body of the tahor that is raising the zav and the cloths only contribute, they would not be tameh midras.
In defense of the Rosh, he explains that this case is different. In cases of hachra'ah any additional weight will raise the zav further. Consequently, since its effect is real, it is not ignored.2
1 The Mishnah Achrona cites the Bartenura that explains that if a zav shook a beam at one end and cause the tahor at the end to move, the tahor is tameh due to heiset while his clothes are tameh midras. He finds this explanation difficult since it does not appear to fit one of the cases of midras (see 2:4) and directs us to the Tosfot Yom Tov who also raises a number of difficulties.
2 The Mishnah Achrona cites the Rash that brings this distinction and the Rambam who does not.
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