The Mishnah (26:7) teaches the following:
Any [leather kli] that does not lack any work [to complete it], becomes susceptible to tumah through machshava (intention for use), and any [leather kli] that lacks work, machashava does not make it susceptible to tumah -- except for the utzba.
What is the utzbah?
The Bartenura explains that it is a riding cloth that was placed over a saddle or a seat of a wagon. The reason why, even if incomplete, machshava would make it susceptible to tumah is because the utzbah was used, even if the edges were not trimmed. Rashi however understands that the utbah is a spread that is used as a table. The Meiri explains that it was placed on the floor.
At first glance, the debate may appear to be about the translation of a word with no legal ramifications. Considering that the principle of why machshava is sufficient even if unfinished seems to be the same for both understandings -- the utzbah is used in that unfinished state. Nevertheless, we find the debate is indeed significant.
The Mishnah Achrona, notes that it appears that the principle in our Mishnah applies to all forms of tumah. In other words, for leather keilim to become susceptible to any form of tumah by way of machshava, it must be complete. The Rambam (Hilchot Keilim 24:6) however rules that this Mishnah is only referring to tumat midras. Recall that if a zav or zava sit or lie on an item that is designed for that purpose, it becomes an av ha'tumah (a source of tumah) itself -- this is referred to as tumat midras. With respect to susceptibility to other forms of tumah it appears that machshava is not required.
The Mishnah Achrona however is at a loss to why there should be a difference between tumat midras and other forms of tumah in our Mishnah. Note that the Bartenura that understands that the utzbah is a riding cloth, appears to understand that subject of our Mishnah is susceptibility to tumah midras. According to Rashi however, who understands that the utzbah is a form of table, must understand that the Mishnah is discuss susceptibility to tumah more broadly. We shall try to understand the position of the Rambam.
The Chazon Ish, cites Gemara Zevachim (94a) that teaches that a hide when removed requires machshava. If however it is tanned, it does not require machshava to be susceptible to tumah (see also Shabbat 49b). Why? The Chazon notes that we learnt (24:12) that if one intended to use the leather to make straps, then it is not susceptible to tumah (since it is incomplete). Nevertheless, in general piece of leather were used for wrapping other object. Consequently, simple pieces of leather, are defined as kli and susceptible to tumah even without machshava. We can then understand why it is only with respect to tumat midras, where the kli is designated for lying or sitting upon, that machshava is then required. Since pieces of leather were not generally used for that purpose, machshava to change it from its default use is required.
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