Keilim, Chapter 13, Mishnah 7. This section of the masechet is occupied with utensils that are put together from two different materials, part metal and part wood, and the question of which part determines the halachic identity of the keli. The practical difference is large: a flat metal utensil is susceptible to tumah even without a receptacle, while a flat wooden utensil that holds nothing is not susceptible at all. Our mishnah presents several such combinations.
The kedum from Ashkelon
The first case involves a keli called a kedum. It was a wooden pole fitted with metal hooks at its end. Jars of water were hung from those hooks and left suspended in the open air so that the water inside would cool. These poles were characteristically manufactured in Ashkelon, and so they were known by that name.
The mishnah teaches: "hakidumin ha'Ashkelonin shenishberu, veha'onkeli shelahen kayemet, harei eilu temei'in." Ashkelon poles that broke, but whose hook remains intact, one hook still attached to each pole, remain susceptible to tumah. The reason is that the hook is what gives the utensil its purpose. It is the hook that does the work of holding the jars, and the wooden pole serves it. Once even a single hook is still in place, the keli is treated as a metal utensil, and it can therefore contract tumah even though it is flat and holds nothing.
A wooden fork or comb with one metal tooth
Attention now shifts to implements constructed completely of wood: "hama'aver, vehamizreh, vechen masrek shel rosh." A ma'aver is a pitchfork made of wood. A mizreh is a winnowing fork of wood, employed at the threshing floor to part the grain from the chaff. A masrek shel rosh is the comb a person runs through his hair. Now, what if such an item lost a piece, "shenitlah achat mishinehen," one of its wooden prongs having gone missing, "va'asa'an shel mateches," and the owner then fitted a replacement prong of metal in its place? Concerning all three the verdict is "harei eilu temei'in": each of them is able to receive tumah.
The same principle of identifying the operative component is at work. With a pitchfork or a comb, the prongs are effectively the entire implement, since they are what performs the job. Accordingly, when a single solitary prong is made of metal while the remaining prongs are wood, the whole keli is classified as a metal vessel, and that one prong is what renders it able to contract tumah.
Rabbi Yehoshua's response
Rabbi Yehoshua declines to endorse any of these decisions. The mishnah records his reaction to the entire list: "ve'al kulan amar Rabbi Yehoshua: davar chiddush chiddeshu sofrim." The Sages, he says, have legislated something altogether new here. And he continues: "ve'ein li mah ashiv," meaning that if someone were to come and dispute the ruling, he has no argument at hand with which to uphold it. He cannot locate any rationale in these halachot that he would be prepared to defend, for to his mind they do not correspond to the way such utensils genuinely function.
In his view, all of these utensils are wooden and none of them has a receptacle, and therefore they should simply not be susceptible to tumah. The presence of one metal component, a single remaining hook or a single replacement tooth, is not enough to alter that status. For a utensil like this to be able to contract tumah, according to Rabbi Yehoshua, its metal portion would have to be far more substantial than one hook or one tooth.