Negaim, Chapter 14, Mishnah 5. The purification of the metzora includes two shavings, on the first day of his process and again on the seventh. Our Mishnah steps back from the details and gives us a broad, almost historical survey: who in all of Torah is commanded to remove his hair as part of a process of sanctity?
"Shloshah megalchin vetiglachtan mitzvah": three people shave, and their shaving is a mitzvah. And who are they? "Hanozir, vehametzora, vehalevi'im": the nozir, the metzora, and the Levi'im.
Why the Mishnah says "mitzvah"
Why does the Mishnah bother to tell us that the shaving is a "mitzvah"? That term normally flags a detail that is the ideal way of doing something yet is not indispensable to the result. The Tosfos Yom Tov traces the choice of wording to the nozir. For a nozir, cutting his hair is indeed a mitzvah, but it is not me'akeiv: it does not hold back the conclusion of his nezirus, as Maseches Nazir teaches. Once his korbanos have been offered, wine and tumah are permitted to him even while his hair is still uncut. With the metzora the picture is different, since there the shaving genuinely blocks the process: as long as it has not been done, his taharah cannot proceed.
How the three differ from one another
The Tiferes Yisroel notes that each of the three has something that sets him apart from his companions on the list. The nozir is unlike the other two in that he shaves only the hair of his head, the growth that accumulated during his nezirus. The metzora is unlike the other two in two respects: he requires two separate shavings rather than one, and his shaving must specifically be performed by a kohen, which is not the case by the nozir or by the Levi'im. And the Levi'im stand apart simply because their shaving belonged to one moment in history, in the midbar, when they were inaugurated into their service. What happened then, happened, and there is no ongoing practice attached to it.
Why is Yechezkel not on the list?
The Mishnah Acharonah raises a sharp question. Yechezkel HaNavi was also commanded to remove his hair. Hakadosh Baruch Hu told him, "Kach lecha cherev chadah," take yourself a sharp sword, "veha'avarta al roshecha ve'al zekanecha," and pass it over your head and over your beard. Why does our Mishnah not count that shaving as a fourth?
He offers two answers. Perhaps the Mishnah is listing only shavings written in the Torah itself, and not those recorded in Nevi'im. Alternatively, a shaving that was commanded to one individual alone, never to be repeated, is not the sort of thing a Mishnah bothers to enumerate.
Two requirements: a blade, and no leftover hair
The Mishnah continues: "Vechulan shegilchu shelo beta'ar, o sheshiyeru shtei se'aros, lo asu kelum." All of them, if they shaved without a ta'ar, or if they left over two hairs, have accomplished nothing.
The first condition is that the removal be done with a blade. The reason behind this requirement is not revealed to us, but the halachah is clear that a ta'ar is what the Torah asks for. The second condition is that nothing be left behind. If a person removed all of his hair except for two hairs, the shaving is invalid where shaving is me'akeiv, as by the metzora and by the Levi'im, and those two hairs must still be cut at that point.
The Tosfos Yom Tov draws our attention here to a theme that has run through our entire masechta: "se'ar," hair, in the language of the Mishnah means two hairs. Two hairs are the halachic definition of hair, and that is precisely why leaving two hairs means that the person has not yet removed his hair at all.
A thought on why shaving belongs to all three
Beyond the mechanics of the act, it pays to ask what these three have in common. With the Levi'im the answer lies close to the surface: their shaving in the midbar formed part of raising them to the rank of Levi'im, an act of consecration.
Regarding the nozir there is a familiar dispute as to whether he bears the title chotei. If he does, then his shaving marks the end of the nezirus and is itself a movement forward. If he does not, then as the Ramban explains, the very reason he offers a chatas is that he had been standing on an elevated rung: he had bound his life to the service of Hakadosh Baruch Hu, something praiseworthy, and now he steps down from that height, and the chatas is brought for the descent. Read that way, the test awaiting him afterward is even heavier. Stripped of the safeguards that nezirus provided, he has to carry a measure of that kedushah into everyday living and grow there. Under either reading the tiglaches functions as a doorway: whether he sheds the status of chotei or whether he takes something beautiful and translates it into a world where wine is permitted and contact with a meis cannot be avoided, the shaving readies him for what comes next.
And so too by the metzora. He is, we hope, leaving behind the condition that brought his tzara'as upon him, the motzi shem ra, the ba'al loshon hora, and reentering human society. Here too the tiglaches is an elevating act, the outward mark of a person moving upward.