With the beginning of the sixth perek we learn that if people or keilim support an ohel, then it is only partially considered an ohel. It behaves like an ohel in that it would spread the tumah and make everything beneath it tameh. It however would not act as an ohel to prevent items above the ohel from becoming tameh.
One of the cases discussed in the second Mishnah is a barrel of dried figs or a basket of straw used to fill in a window that is between two ohalot. The Mishnah explains that if the figs or straw rely on the kli from structural support then it is not considered as if the windows has been sealed. If however the figs and straw can stand without the kli, then the windows is considered sealed and the tumat hamet would not pass from one ohel to the next.
The Bartenura explains that in this case the figs and straw had spoiled such that the were not fit for consumption. Since one has resolved to leave them there, they can act as a mechitzah (partition).
The Mishnah Achrona however adds, that even if it were fit for consumption, if one were mevatel (annulled) them, it would also act as a separation (see 15:7). The reason why the Gemara mentions that they are not fit for consumption is because in that case, bitul is assumed and need not be declared.
The Mishnah Achrona continues, citing the Tosfot (Bava Batra 20a), that we find the requirement of bitul only when attempting to reduce the space of the window to less than a tephach. If however one completely seals the window, bitul is not required. What is the basis if this distinction?
The Mishnah Achrona suggest that the requirement of bitul is rabbinic. This is out concern that one may take the item that reduced the space of the window to less than tephach and continue under the incorrect assumption that tumah could not spread from one room to the next. Since the change in dimension may be difficult to appreciate, there is a concern that one would think that nothing had legally changed, when in truth the size is now greater than a tephach. In the case however where the window was completely sealed there is no reason for concern. Once the object has been removed, the window between the two ohalat is now open, so one would suspect that perhaps the legal status has changed.
The Grach (281) however provides a different explanation. He begins by questioning why the minimum size of a window for tumah to pass through is a tephach. Is it because it is like any gap or cavity between two ohalot, that once it is a tephach wide tumah can pass through? Or is this a specific law for a window, that if it is less than a tephach, tumah cannot pass through. He concludes that it must be the latter, for we will learn (13:1) that there are certain cases (e.g. if the window was made for lighting) where the gap can be smaller and tumah can still pass through, or even larger and it will not. In other words, depending on its use and origin, the size of a window to be defined as a petach will change. It happens to be that in our case it is a tephach.
Based on this understanding the Grach explains that if one placed something in the window, thereby reducing the space, unless he is mevatel, its presence does not impact its status as a petach -- it is simply like a package in the doorway. Once the individual is mevatel, then the window losses its status as a petach and what remains is simply a hole. If however one completely seals the window, then bitul is not necessary, since it is not an issue of removing the status of a petach from the window. This is because the full seal acts as a mechitzah preventing the tumah from reaching the window at all.
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