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The eighteen perek begins by once again discussing the shida -- a large wooden chest. Recall that we learnt in the fifteenth perek that if the shida can contain forty seah of liquids or 2 kor of dry goods (three square amot in volume) then it is not susceptible to tumah. Our Mishnah records the debate regard how this volume is calculated. The opinions range from the considering only the contents, to including the walls and even the space beneath the shida if the legs are shorter than one amah. We shall try to understand this law, considering the debate in our Mishnah.
The Torah lists a number of different keilim when teaching that vessels can become tameh: "... whether it is a wooden utensil, a garment, leather or sackcloth -- any utensil with which work is done -- shall be brought into water and remain contaminated until evening and then become cleansed" (Vayikra 11:32). The Chachamim note that sackcloth is mentioned in the list. Since vessels made of sackcloth can be used and moved both empty and full, the Chachamim understood that for a vessel made of the other materials listed in the pasuk, it must be able to be moved both empty and full to be susceptible to tumah. Once the volume reaches forty seah, this requirement would not be satisfied and those keilim would not be susceptible to tumah.
The Rambam (Keilim 3:1) explains that the susceptibility to tumah hinges on the ability for the kli to be moved (empty and full). Therefore, if a wooden kli is designed to stay put, then it is not susceptible to tumah irrespective of its volume. Similarly, if it is moved when full, then it is susceptible to tumah even if it is exceptionally large. The Rambam understands that when the Mishnah provides a shiur (a measure) of forty seah, it is refers to general wooden keilim, since it is assumed that once it gets to that volume it will not be moved.
The Raavad, however argues that the shiur is what determines the susceptibility to tumah. The only small wooden keilim that would not be susceptible to tumah is if it is forbidden for it to be moved (e.g. the mizbeach ha'zahav). The only large keilim that would be susceptible to tumah are those designed to be move when loaded (like those listed in the beginning of the fifteenth perek).
The Chazon Ish (Keilm 24:1) notes that if the determining factor was whether the kli was movable, then the debate in our Mishnah would not make much sense. For example, how can we understand the position of Beit Shamai that does not include the thickness of the walls. Considering that the walls would contribute to the weight of the kli and whether it can be moved, surely they should be included.
Consequently, the Chazon Ish understands that the forty seah requirement is another shiur of the Chachamim. In other words, much like all shiurim, the Chachimim were required to provide a fixed measure to define different activities. For example, how much food is considered eating? Similarly in our Mishnah the Chachamim determined that after a particular volume, even if the kli could physically be moved when full, it is not legally considered that the kli is movable due to its awkwardness, and therefore not susceptible to tumah. Once that shiur is in hand, then there is room for the Tanaim in our Misnah to debate how that shiur is measured.
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