Keilim, Chapter 12, Mishnah 6. This mishnah gathers four separate disputes into one list, all of them between Rabban Gamliel and the chachamim, and all of them turning on the same basic question: when does an object, or a part of an object, count as a keli that is capable of receiving tum'ah?
The list is introduced with a count: "arba'ah devarim," four items, regarding which Rabban Gamliel imposes tum'ah, "metamei," whereas the sages, "vachachamim metaherin," hold every one of them to be pure.
The cover of a metal basket
The opening case has already come up earlier in the masechet: "kisui teni," the covering of a teni, "shel matechet," one made of metal, "shel ba'alei batim," of the sort kept by ordinary householders. A teni was a sizable metal container in which people stored miscellaneous odds and ends, bits of scrap metal or fragments of pottery among them. The dispute here revolves around its lid at a time when it stands apart from the container.
Rabban Gamliel rules that such a cover can become tamei even on its own. True, it has no independent name of its own; it is simply "the cover of the teni." But in practice people do put things into it and use it by itself, and that gives it the standing of a keli in its own right.
The chachamim disagree. The fact that the cover is physically capable of holding things is not the deciding factor. It was not fashioned for that purpose, and an item that was not designated for receiving is not a receptacle. Therefore it remains tahor.
The loop of the bathhouse scrapers
The second dispute concerns "telui hamagredot," the hanging loop attached to the scrapers people used when bathing. These scrapers were drawn across the skin to clean it, and a loop was fastened to them so that they could be hung on the hooks in the bathhouse.
As a rule, a loop of this sort does not become tamei even while it is attached to a keli, since it contributes nothing to the work the keli actually performs; it serves only for hanging the item up. Rabban Gamliel, however, holds that here the loop does improve the use of the scraper itself, because it gives the user a better grip. It is therefore treated as part of the scraper and becomes tamei along with it.
The chachamim maintain that the loop has no bearing on the scraping at all. Its whole purpose is to hang the scraper on a hook, and since it adds nothing to the utensil's usefulness, it stays tahor.
Unfinished metal utensils
The third case is "galmei klei matachot," metal utensils in an unfinished state. These are vessels that have reached the stage where they can already be used, but the work on them is not complete: they have not been polished or ornamented, or some component is still missing.
Rabban Gamliel holds that since they are already serviceable to some degree, they are kelim and are susceptible to tum'ah. The chachamim hold that as long as the manufacture is not complete, the object has not yet attained the status of a keli, and it therefore cannot become tamei.
A tray broken into two equal pieces
The last of the four is "tavlah shenechlekah lishnayim": a pottery serving board that came apart into two fragments which look to be the same size.
According to Rabban Gamliel, both halves are susceptible to tum'ah. His argument runs like this: the two may appear identical, but no break ever yields portions of precisely matching size, so one of them is necessarily bigger. Whichever piece that is holds most of what the tray once was, and it therefore keeps its earlier standing as a keli beyond any question. Since there is no way to identify which one it is, each is handled as though it were the greater piece.
The chachamim rule that neither fragment can become tamei. People do not generally continue using a broken tray unless it is clear to them that they are holding the larger portion; otherwise they simply discard the pieces. Consequently, when it cannot be told which piece is bigger, neither of them retains the standing of a keli, and both are tahor.
Where they agree
The mishnah ends with a case in which the sages grant Rabban Gamliel's point regarding one of the pieces: "umodim chachamim leRabban Gamliel," the sages concede, in the situation of "betavlah shenechlekah lishnayim," a tray split in two, where the fragments are "echad gadol ve'echad katan," one of them bigger and one smaller, that "shehagadol tamei," the bigger fragment does contract tum'ah, "vehakatan tahor," while the smaller one does not.
In such a case nothing is left uncertain. It is plain to the eye that the bigger shard makes up most of what the tray had been, so it goes on serving as a utensil and people continue to use it. Just as plainly, the smaller shard is only the leftover minority; nobody bothers with it, so it never counts as a keli and stays tahor.