Negaim, Chapter 14, Mishnah 7. The taharah of a metzora requires a set of items: two birds, cedar wood, a strip of crimson wool, and hyssop. This mishnah turns to two of them and defines what qualifies: the piece of eitz erez (cedar) and the eizov (hyssop).
The dimensions of the cedar
The mishnah opens with the words "Mitzvas eitz erez", the requirement governing the cedar. Regarding its length it states "orko amah", one amah from end to end. Regarding its girth it offers a comparison: "ve'ovyo kerevia kera hamitah", one fourth of the width of the leg of a bed. The mishnah does not leave that fraction for us to work out on our own; it spells out how to reach it: "echad lishnayim ushnayim le'arbah", split the leg once so that it becomes two, and split those two again so that they become four.
The Rav explains that this description was formulated in Bavel: you take the leg of a bed, split it in half, and then split that half again in half. The result is a quarter of the original bedpost. Why did the mishnah bother to walk us through the splitting? To teach that here the mitzvah is letzamtzeim, to hit the measurement exactly. It must be precisely a quarter of the thickness of a bed leg, no more and no less.
Which hyssop is fit
Turning to the second item, the mishnah opens with the words "Mitzvas eizov" and then proceeds by way of exclusion, listing the kinds of hyssop that cannot serve. Ruled out are eizov Yavan, the Greek variety; eizov kochli, the blue one; eizov Romi, the Roman one; and eizov midboris, the sort that grows in the wilderness. The list then closes with a sweeping rule: "velo kol eizov", and no hyssop at all, "sheyeish lo sheim livoy", that comes along with a companion word appended to its name.
What is a sheim livoy? A name that accompanies the plant's name and links it to something else, exactly as in the mishnah's own examples: Greek hyssop, blue hyssop, Roman hyssop. The principle is familiar from the Gemara in Sukkah regarding the hadas shoteh, which is disqualified for the lulav because of the arrangement of its leaves. Its very name announces that it is not the genuine hadas. So too here: once a plant needs a qualifying word tacked on to its name, that word itself testifies that this is not the plain eizov of which the Torah speaks.
A question of geography
The Tiferes Yisroel raises an interesting difficulty. The Rambam rules that those portions of the taharah of a metzora which do not involve korbanos, the parts that can be carried out away from the Beis Hamikdash, are in force even in chutz la'aretz. This is not a mitzvah bound up with the Land of Israel; it is an obligation that rests upon the metzora himself, wherever he happens to live.
If so, asks the Tiferes Yisroel, what about a metzora living in Rome? There, nobody calls the local plant "Roman hyssop"; they simply call it eizov. The same question applies to one living in Greece, where the eizov Yevoni is referred to by everyone as plain hyssop. Should the local name settle the matter? His conclusion is that it does not. The mishnah is teaching that even so, the varieties it lists, Greek, blue, Roman and wilderness hyssop, remain permanently unfit, in their own places as well.
Why the mishnah omits some measurements
The Tiferes Yisroel presses further: why did the mishnah not record the size of the eizov? The halacha, as the Gemara in Niddah teaches, is that its length is a tefach, a handbreadth. And why did the mishnah not mention the weight of the crimson wool, which the Gemara in Yoma sets at a shekel of silver?
His answer: every measurement that our mishnayos record is one that was received at Sinai as an exact figure, and therefore must be neither less nor more. The cedar's thickness belongs in that category, which is why the mishnah insists on a precise quarter. The eizov and the lashon shel zehoris, by contrast, are different. There they may be increased, since no tradition fixed their exact size. All that was given is a floor: the eizov may not be less than a tefach, and the crimson wool may not weigh less than a shekel. Because these are minimums rather than exact quantities, the mishnah does not present them among its measurements.
So this mishnah gives us two lessons in precision and identity: the cedar must be measured exactly, and the hyssop must be genuine hyssop, plain and unqualified, with no second name clinging to it.