Negaim, Chapter 4, Mishnah 2. The Rambam opens this mishnah with a useful signpost: everything taught until this point was devoted to cataloguing the simanim, the marks by which tumas tzara'as is confirmed, and from here the Tanna shifts to laying out the specific halachos that belong to each siman on its own.
It pays to refresh the list of categories. On the human being, tzara'as can appear in four places. One is or basar, plain skin of the flesh. Another is shechin and michvah, skin damaged or raw from a burn, from a wound, or from sickness. A third is karachas and gabachas, the rear and the front of the head of someone whose hair has fallen out. A fourth is the niskei adam, the places where hair grows, namely the rosh and the zakan, the head and the beard. Away from the body there are nigei begadim, clothing that turns yerakrak or adamdam, intensely green or intensely red, and nigei batim, houses, which also turn intensely green or intensely red. That yields six categories of this tumah in all: four upon a person, one in clothing, one in a building.
Within or basar there are three simanei tumah: se'ar lavan, a pair of dark hairs that have gone white; michyah, an island of sound flesh inside the white area; and pisyon, the growth of the nega beyond its original borders, reckoned only from the close of the first seven day period onward. Our mishnah selects two of these three, se'ar lavan and pisyon, and sets them side by side. In the Rambam's formulation, each of the two carries stringencies that the other lacks.
The stringencies of se'ar lavan
"Yeish bese'ar lavan," there are features in white hair, "mah she'ein bepisyon," that spreading does not have, "veyeish bepisyon," and spreading has features, "mah she'ein bese'ar lavan," that white hair does not have.
The first strength of se'ar lavan: "shese'ar lavan metamei bis'chilah," it conveys tumah from the very first examination. Suppose a man discovers a white blotch on his skin in one of the four shades enumerated at the start of the tractate, or in one of those shades mixed with red. The four are the white of snow, the lime plaster on the walls of the Beis Hamikdash, the clean wool of a lamb on the day of its washing, and the fine membrane found inside an egg. Should the kohen, at that very first look, find two dark hairs inside the blotch that have turned white, the man is pronounced muchlat immediately, a metzora in full, and nothing further is examined. Spreading behaves otherwise. Even were the kohen present on day one and able to watch the nega enlarge before him, growth carries no weight at the beginning. It counts once the first seven days have run their course, or once the second seven days have run their course, or with houses after a third such period, and later on when a nega already ruled tahor grows once more.
The second strength: "umitamei bechol mareh lavan," white hair conveys tumah in the company of any grade of white at all. The Rav explains that this takes in whites fainter than the palest of the four maros, fainter, that is, than an egg's inner membrane, and even bohak. On its own such a washed out white would never bring the man into a status of tumah, yet once two dark hairs inside it have turned white, its faintness is irrelevant. The Rav anchors this in the language of the pasuk, "umareh hanega amok," the appearance of the nega is deep. Intensity in this area is described in terms of depth: a stronger white looks deeper, and a weaker white looks raised and shallow. The Torah demands that depth, that intensity, of the nega; it never states that the se'ar lavan must present a deep appearance. Consequently the white in which the changed hairs sit need not attain the strength of the four shades. With spreading it is otherwise: growth in a white paler than the egg membrane, the weakest of the four, does not convey tumah.
The third strength: "ve'ein bo siman taharah," white hair holds nothing within it that can produce taharah. Spreading does hold such a possibility, for once the growth has covered the man from head to foot, the metzora is tahor.
The stringencies of pisyon
"Shehapisyon metamei bechol shehu." Growth conveys tumah in the tiniest quantity. At the opening examination the kohen traces the outline of the nega with ink or with some other pigment, fixing its border. If upon his return a week later the nega has crossed that mark by even a hair's worth, less than the area of a lentil, that minute increase is pisyon and it renders the man tamei.
"Umitamei bechol hanega'im," growth conveys tumah in every one of the categories of nega. And it conveys tumah chutz lanega, outside the nega, past the om, the original body of the blotch, since spreading by its nature reaches beyond it. "Mah she'ein kein bese'ar lavan," and here white hair differs: the changed hairs have to be positioned within the white patch itself.
Why se'ar lavan means two hairs
The Tosfos Yom Tov confronts a question that lies beneath this entire discussion. Throughout, we take for granted that se'ar lavan requires two hairs rather than one. On what basis? He examines the two forms of the word in Hebrew, se'ar and sa'arah, and assembles a variety of commentators who employ that distinction, some of whom maintain that the term se'ar by its nature signifies a pair, and he follows the usages through Tanach. Among the works he draws upon are the Mikneh Avram, the Maharal of Prague's Gur Aryeh, and the Ralbag.
He then brings the Maharam, evidently the Maharam of Lublin, whose glosses are printed beside the Maharsha in the Gemara and whom the Tosfos Yom Tov quotes elsewhere in this tractate too. The Maharam states that he has no source for the requirement of a pair, and then proposes a reading based on the words of the pasuk, "use'arah lo hafach lavan." Had Scripture intended one single hair of the nega, the word would have needed a mappik hei, yielding the sense "its hair did not turn white." Since the hei stands there without a mappik, the term refers to hair as a collective, hence more than a single strand. If one hair were enough, the verse ought to have used either a mappik hei or the plain form se'ar with no hei at all. The choice of se'arah with an unpointed hei indicates plurality, two hairs.
As an aside, it is this same Maharam who settles the difficulty in nigei batim regarding whether a window is cut into the wall for the inspection: the Mishnah implies that it is not, while the Toras Kohanim implies that it is, and he draws a line between an unbroken wall, into which no opening is made, and a wall that already contains a window, where one is made.
The mishnah thus leaves us holding a scale in balance. Se'ar lavan is the harsher siman in that its tumah takes effect at once, it operates alongside white of any intensity, and it offers no route to taharah. Pisyon is the harsher siman in that even a minimal amount suffices, it functions in every type of nega, and its tumah works past the border of the nega itself.