Ordinarily, nails are not susceptible to tumah. The Tifferet Yisrael explains that since they are generally designed to be driven into walls and walls are fixed in the ground, they are not susceptible to tumah.*The Mishnah (12:5) teaches that if one modifies (hitkin) a nail to be used for locking and unlocking, then it is not susceptible to tumah. The Bartenura explains that the person either bent or sharpened the nail for this purposed. The question that needs addressing is if no physical modifications are required, is setting it aside for that purpose enough to make the nail susceptible to tumah*.
The Tifferet Yisrael notes that we learn later (25:9) that all utensils can become susceptible to tumah by mental designation alone – a physical change is not required. Nevertheless, citing the Tosfot (Bava Metzia 52b), he explains that if the kli requires a modification, then mental designation alone does not suffice. Consequently, we must understand that according to the Bartenura, the bending and/or sharpening is necessary to make the nail functional.
The Mishnah Achrona however notes that there is a debate between R’ Akiva and the Chachamim regarding a nail that has been set aside for opening barrels. R’ Akiva understands that the designation is enough to make this nail a kli whereas the Chachamim maintain it must be fired first, an action must be taken, for the nail to become susceptible to tumah. One would expect that they would be arguing in both cases, yet the Mishnah only records the debate regarding the latter.
One possible answer is that the difference between the two cases is that in the latter, the nail could already be used to open the barrel if really necessary. Consequently the debate is only regarding cases where the kli could just be used, albeit in a substandard manner, without modification. In the early case however, the nail unmodified cannot function as the key at all.
Yet the Mishnah Achrona finds this fine distinction very difficult to accept. Furthermore the language of “hitkinu”*in the first case is understood to mean it involved modification, while in the second case, the word is associated with the opinion of R’ Akiva and is understood as meaning the nail is simply designated for that purpose without and physical change. The Mishnah Achrona therefore understands that the word hitkinu in both cases must mean that the nail has been set aside without any modifications. He finds support for this understanding in the Rambam where he codifies the law in this Mishnah*.
We do find however the word hitkinu used twice in the later Mishnah (12:7). There it is regarding a coin that has become invalid. The Mishnah explains that it if it reused as a necklace or weight then it susceptible to tumah. The Mishnah Achrona notes that the Tosfot cited previously understands that the hitkinu here means that the coin is punctured so it could be used as a necklace. The Tifferet Yisrael however notes that the second reference, with respect to using the coin as a weight, does not mean that a physical change is necessary and designation alone would suffice to make it susceptible to tumah. Consequently, according to the Tifferet Yisrael the change in meaning of hitkinu is acceptable and should not present a difficulty in understanding our Mishnah.
The Mishnah Achrona however draws a difference between that Mishnah and ours. He explains citing another Tosfot (Shabbat 49) that designation alone is only enough if we are dealing with a kli that is not susceptible to tumah. For example, an animal’s ring that is going to now be used for human. If however object is not a kli at all, then a physical change is required. He understands that these coins are such a case. Consequently a physical change would be required for the coin whether it will be used as a necklace or weight. (The Mishnah Achrona raises this Tosfot from Shabbat as question on the Tosfot in Bava Metzia who does not apply this logic to the case of the coins.) The term hitkinu is therefore consistent in that Mishnah. However we are still left with the question that, according to the Mishnah Achrona the term means different things in Mishnah 5 and 7.
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