The twentieth perek begins by teaching that even if a pillow or blanket tears and can no longer contain its stuffing, it is susceptible to tumat midras. The Bartenura explains that there keilim are design to be fill and lied upon. As we have discussed previously, if a kli is designed to contain items, it is susceptible to different types of tumuh (tumat met, tumat sheretz, etc). If it is designed to be lied upon, then it is susceptible to tumat midras. This means that if a zav or zava sits or lies on the kli, not only does it make the kli tameh, but it makes it an av hatumah, such that it can cause people or other keilim to become tameh.
The Bartenura explains that since the pillow and mattress' prime design was for lying upon, despite no longer being able to contain the stuffing, it is susceptible to tumat midras. The Tosfot Yom Tov cites the Rambam and explains, that despite it no longer being susceptible to tumah originating from a dead sheretz, it can become tameh by way of tumat midras.
The Tifferet Yisrael notes that this Mishnah should not be confused with the earlier one (19:9) where we discuss the chest that has its opening in the side and suitable for sitting on. That chest was susceptible to tumat ha'met and tumat midras. If however the base of the chest broke, the Chachamim maintained that since its prime use could not longer be met (it can no longer contain anything) it is not susceptible to any form of tumah, despite still being fit for sitting on. The Tifferet Yisrael explains that the chest's prime use is for storing items, and its capacity to act as a seat is clearly secondary. In our case however, the mattress and pillow's prime use from the outset was for lying on and containing the stuffing.
One question raised on the position of the Rambam is that we have a principle, that any kli that is susceptible to tumat midras, is susceptible to other forms of tumah. It is therefore surprising that the Rambam rules that in our case, the keilim are susceptible to tumat midras and not tumat sheretz.
The Chidushei Mahariach suggests that that principle only applies to keilim. Once the mattress tears it is no longer considered a kli. Nevertheless since it was designed for lying on from the outset, it is susceptible to tumat midras.
The Aruch HaShulchan (Keilim 228:23), explains similarly citing the Tosfot Yom Tov (24:4). The principle does not apply to broken keilim. In our case, the kli could act as a receptacle and was susceptible to all type of tumah. Once it broke such that it could not act as a receptacle, it is no longer susceptible to those forms of tumah. The principle only applies to keilim and that are still of the form in which they were designed.
To understand this distinction, the Aruch HaShulchan directs us to his early comment (227:1-4). The Rambam rules (23:1) regarding a mat that while it is susceptible to tumat midras, it is susceptible to other forms of tumah rabbinically. In other words, the principle that anything that is susceptible to tumat midras is susceptible to other forms of tumah is a rabbinic principle. The Aruch HaShulchan clarifies that this is only in the case where, like in the case of a mat, it cannot act as a receptacle. Returning to the Tosfot Yom Tov, we can only accept his explanation, that the principle does not apply when the mattress tear open, if the principle itself is of a rabbinic nature and the Chachamim simply did not apply the gezeira in that case.
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