At the beginning of the second chapter of Keilim we learn:
Wooden utensils... when flat are pure, and when containers are impure.
In other words: only wooden vessels that can serve as receptacles can contract impurity, while if they are flat they are not susceptible to impurity - like vessels made of rock. (The exception is a wooden mattress or any other wooden objects one commonly lies on, the source of which is a unique pasuk).
While this seems quite clear, many Rishonim disagree and the issue of the possible impurity of wooden vessels becomes a matter of great debate. Let us mention 3 sources, from which one can imply that wooden vessels can and indeed become impure:
The Gemara in Bava Batra (66a) tells us that flat wooden utensils contract impurity by way of rabbinic decree. So although there is no Torah-based impurity, there is a rabbinically-based one.
There is a discussion in the Torat Kohanim which deals with the issue of flat wooden utensils, and states that although we generally hold that they do not contract impurity, if they serve things that serve man (like a table facilitates a plate used by people) - they also can become impure.
The Tosafot (Sukkah 16a) and Ritva (Sukkah 12b) infer from Mishnah (18:9) that wooden flat utensils can become impure. The Mishnah teaches that a bed becomes pure and impure in the same way: Meaning, if the bed became impure fully constructed it can only be purified in the same way, and if it became impure when it was disassembled then it can only become pure in that way. Since beds are made of wood, it is interesting that we learn that even when it is not assembled - hence: not usable - it still can become impure.23
Generally, we find three opinions in the Rishonim regarding what the Halacha is:
a. The Rashbam claims that there is no impurity for wooden utensils, at all. The Gemara in Bava Batra is according to a singular opinion; the Torat Kohanim is overruled by the Gemara; and our Mishnah can be explained as talking about a bed made from other materials.
b. The Tosafot adopt the view of the Torat Kohanim, as stated earlier.
c. Rambam appears to interpret the Torat Kohanim in saying that any wide wooden utensil can become rabbinically impure.
23 Ed Note: This is one understanding of our Mishnah based on the Tosefta. Others explain the basic meaning of the Mishnah differently. (See the Tifferet Yisrael, for example, for more detail.)
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