Keilim, Chapter 4, Mishnah 4. Understanding this mishnah requires one law that was already laid down in the second perek: clay vessels are governed by their own set of rules regarding tumah. Should a tamei object come into contact with the outer surface of an earthenware vessel, the vessel is unaffected and remains tahor. Only when the tumah passes into the hollow space within its walls does the vessel contract tumah. With clay, the cavity is decisive and the outer wall is irrelevant.
Applying that law is straightforward as long as the vessel has a single ordinary opening. The difficulty arises when the upper portion of the vessel is divided into several distinct troughs or channels, so that tumah can come to rest in one of them while never reaching the rest. Which of these spaces has the status of the vessel's cavity, and which is no more than a ledge on its exterior? This is precisely what our mishnah comes to clarify.
A vessel with three rims
The mishnah opens with the case: "kli cheres sheyesh bo shalosh sefayot," a clay vessel fitted with three rims. Imagine one rim surrounding the actual mouth of the vessel, with two further rims built around it in concentric rings, each rim separated from the next by an open channel. Tumah now settles in one of those channels and nowhere else. Has the vessel become tamei?
The entire ruling turns on which of the three rims is the tallest, since the highest rim is what marks off where the vessel's cavity ends.
The inner rim is highest
"Hapenimit odefet": the innermost rim towers over the other two. Tumah now comes to rest somewhere beyond that inner rim, either in the channel separating it from the middle rim, or in the channel between the middle and outermost rims. The verdict is "hakol tahor": nothing has become tamei, neither the vessel nor the surrounding rims.
This follows straight from the law with which we began. Because the innermost rim towers highest, only the area it encircles is treated as the vessel's cavity. Anything lying past that rim is merely the exterior of the vessel, and an earthenware vessel does not contract tumah through its exterior.
The outer rim is highest
"Hachitzonah odefet": here it is the outermost rim that rises above the rest. Now, should tumah settle anywhere in the space bounded by that outer rim, "hakol tamei": all of it is tamei. Since the tallest of the three is the outermost, everything it encircles, the channels included, is reckoned as the vessel's cavity. Tumah that comes to rest anywhere within that perimeter has effectively entered the hollow of the vessel.
The middle rim is highest
"Ha'emtza'it odefet": and what if the middle rim is the one that stands above its neighbors? Here the mishnah gives a divided ruling: "mimenah velifnim tamei, mimenah velachutz tahor." Tumah that settles on the inner side of the middle rim renders the vessel tamei; tumah that settles on its outer side leaves the vessel tahor. Once more, the tallest rim is where the line is drawn: whatever the middle rim encloses is the cavity, and whatever lies past it is no more than the vessel's outer surface.
All three rims are equal
"Hayu shavot," the three rims were built to the same height. With none of them projecting above the others, where is the boundary of the vessel's cavity to be placed?
"Rabbi Yehudah omer, cholkin ha'emtza'it." Rabbi Yehudah rules that the middle channel, the one running between the innermost and middle rims, is split down the middle. Its inner portion is reckoned as belonging to the vessel's cavity, so tumah settling there makes the whole vessel tamei. Its outer portion is reckoned as exterior, so tumah settling there leaves the vessel tahor.
"Vachachamim omrim, hakol tahor." The Chachamim hold otherwise. To their mind no channel gets divided at all. The cavity of the vessel is limited to what the innermost rim encloses, and consequently tumah anywhere past that rim leaves the vessel tahor.
When does an earthenware vessel become susceptible at all?
The mishnah concludes with a broad rule governing clay vessels: "klei cheres mei'eimatai mekablin tumah? Misheyitzarefu bakivshan, vehi gemar melachtan." At what point do earthenware vessels become capable of contracting tumah? Once they have been hardened in the kiln, since that firing constitutes the completion of their production.
Before the fire has set the clay, there is no vessel here whatsoever. Once the piece has emerged from the kivshan, its manufacture is considered concluded. This is so even where the potter still plans additional labor on it: as far as tumah is concerned, the baking marks the finish line. From that moment the piece counts as a finished vessel and can receive tumah.
Our mishnah has thus drawn two lines. One is a line in space: the highest rim determines where the cavity of a clay vessel begins, and tumah can take effect only there. The other is a line in time: the kiln determines the moment at which shaped clay becomes a vessel in the eyes of halachah.