One of the unique ways in which a zav can transfer tumah is tumat mishkav. In other words a zav can transfer tumah to an object usually fit for sitting on by standing, lying, sitting or leaning on it; or by being raised when the object acts as a counter-weight. When tumah is transferred, that item becomes an av ha’tumah like the zav himself. An additional novelty is that if the zav sat on a pile of such objects they would all become avot ha’tumah. A case that is debated in the Mishnah (4:7) is where a zav sat on a bed and under each leg of the bed was a cloak. The Chachamim ruled that in that case each of the garments is tameh while R’ Shimon maintained that each of them is tahor. What is at the core of the debate?
This Mishnah is raised in Gemara Shabbat (93a) in the context of another discussion. The question raised is regarding the liability of two people that performed one melacha on Shabbat. More specifically, if one of the men only assisted another that does not need help, then has he transgressed a biblical prohibition? The Gemara concludes that a mesayeh (one who assists) is exempt (even though rabbinically prohibited).
To elaborate further, the Mishnah (Shabbat 10:5) explains that if two people perform a melacha that each could perform on their own then they are exempt on a biblical level (nevertheless rabbinically prohibited). If however the melacha requires both of them (ze eino yachol ve’ze eino yachol), for example carrying a large couch, then there is a debate. The Chachamim rule that they are both chayav since it is as if each of them performed the melacha independently. R’ Shimon however argues that both are exempt since each is considered only as assisting the other and not performing the complete melacha.
Rashi (ibid.) explains that that argument is behind our Mishnah. In other words, in the case of the bed each leg on its own cannot support the zav - all are required. Consequently the Chachamim maintain it is as if each of them is independently supporting the zav while R’ Shimon understands that each leg is considered as only “assisting the other” and therefore “patur” – not assisting at all.
The Tosfot sighting the Rashba explains that the explanation provided by Rashi is not necessary, as this debate merely reflects an early one. The Mishnah (4:5) had discussed a case where a zav sat on scales and was raised by the weight of a number of items on the other side. The Chachamim ruled in that case that they are all tameh mishkav while R’ Shimon disagreed. R’ Shimon there argues that the reason that they are all tahor is because it was only their combined weight that raised the zav. R’ Shimon explains that he requires one of the items to bare the majority of the weight. Consequently in our case as well since none of the garments beneath the bed bares the majority of the weight, none are tameh.
The Sfat Emet defends Rashi explaining that Rashi certainly agrees that in our Mishnah as well R’ Shimon requires that one of the garments bare a majority of the weight. Nevertheless had he agreed with the Chachamim in the case of ze eino yachol ve’ze eino yachol then it would be considered as if each of the garments is bearing the full weight of the zav. Accordingly Rashi is explaining that R’ Shimon only deems the clothes tahor because he requires them to hold most of the weight, and that even though each cannot hold the zav on their own, it is only considered mesayeh and not as if they are each independently holding the weight. As a further support to this explanation, perhaps this is why the debate appears to be recorded twice so that both this laws can be taught.
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