Shabbos, Chapter Two, Mishnah 3. This mishnah continues the discussion of which wicks may be used for lighting Shabbos candles.
"Kol hayotzei min ha'etz ein madlikin bo ela pishtan":
Anything that comes from a tree may not be used as a wick, and there is only one exception - flax. In fact, flax itself comes from a seed and is a vegetable, and for that reason grapevine, cotton, hemp and similar materials are fit for use, since they are not considered wood.
Why, then, is flax considered wood? This is derived from the verse in the book of Yehoshua: "Vatitmenem b'pishtei ha'etz" - Rachav hid the spies sent by Yehoshua among the stalks of flax, literally "the flax of the tree." Even though technically it is not wood, since Scripture calls it wood, we treat it as wood, and it is the only type of "wood" with which one may light.
Tumah of an ohel:
From here the mishnah moves to another topic connected to wood: "V'chol hayotzei min ha'etz eino metamei tumas ohel" - anything that comes from a tree does not become tamei with the tumah of an ohel. When a corpse is inside a house, even though everything in that ohel and under that roof becomes tamei, the roof itself and the structure itself do not become tamei, because they are attached to the ground.
Here too the mishnah establishes one exception: "ela pishtan" - except for flax. Since the Torah identifies flax as wood, we too call it wood. Therefore a wooden structure containing a corpse does not itself become tamei (even though the objects inside it do), whereas a tent made of flax that contains a corpse does become tamei.
A wick of cloth - the dispute between Rabbi Eliezer and Rabbi Akiva:
The mishnah returns to the subject of wicks and discusses "pesilas habeged shekiplah v'lo hivhavah" - a cloth wick that one folded but did not singe. It was customary to singe the wick to make it easier to light, and this practice has been preserved to this day, that the husband singes the candles before Shabbos. In this case, where one folded it but did not singe it, the Tannaim disagree:
Rabbi Eliezer says: "temei'ah v'ein madlikin bah" - it is tamei and one may not light with it. The folding alone does not remove its status as a garment, and as long as it has not been singed it retains its status as a utensil that can receive tumah (and if it was tamei, it remains tamei). Consequently one may also not light with it.
Rabbi Akiva says: "tehorah u'madlikin bah" - it is tahor and one may light with it. Once he folded it into a wick it lost its status as a garment, and therefore it is tahor and it is permitted to light with it.
The Gemara's explanation of the dispute:
The Gemara explains the dispute in a complex way, built on four underlying assumptions:
We are dealing with a cloth measuring three fingerbreadths by three fingerbreadths - the minimum size for a piece of cloth to be considered a utensil.
We are dealing with an erev Shabbos that falls on Yom Tov.
We follow the opinion of Rabbi Yehudah, that on Yom Tov one may burn whole utensils, but one may not burn broken utensils. A utensil that entered Yom Tov whole and broke during the day is considered muktzeh and one may not light with it.
One who lights the wick of a Shabbos candle must light the greater part of the wick that protrudes from the lamp, and must hold the flame there until most of it has caught.
According to Rabbi Eliezer, even though he folded the cloth into a wick before Yom Tov, the folding does not remove its status as a utensil. Therefore, when he comes to light it on Yom Tov he will have to hold the flame there until most of it catches, and at that very moment, in the course of the lighting, the wick becomes a broken utensil. It turns out that he is continuing to light a broken utensil, and that is forbidden. For this reason he ruled: it is tamei and one may not light with it.
Rabbi Akiva, on the other hand, holds that the folding done before Yom Tov removes from it the status of a garment. Already before Yom Tov began it is tahor and is not considered a garment, but rather a broken utensil that broke while it was still day, and with such a broken utensil one may light on Yom Tov. For this reason he ruled: it is tahor and one may light with it.
It should be added that the assumption that the folding was done before Yom Tov is based on the fact that one does not make wicks on Yom Tov, and therefore it was presumably done beforehand.
In summary: In this mishnah we learned that anything that comes from a tree is unfit for a wick, with flax as the exception, since it is called wood on the strength of the verse "vatitmenem b'pishtei ha'etz"; that anything that comes from a tree does not become tamei with the tumah of an ohel, and here too flax is the exception; and the dispute between Rabbi Eliezer and Rabbi Akiva regarding a cloth wick that was folded but not singed - whether the folding removes from it the status of a garment, with the resulting implications for its tumah and for whether one may light with it.