Negaim, Chapter 4, Mishnah 3. The opening chapters laid out the signs through which tumas tzara'as is determined; from here the Tanna turns to the distinctive rules that belong to each sign, setting them side by side to see where one is stricter than the other.
A word of orientation. Tzara'as can appear on a person in four different locations: the ordinary skin of the body (or basar); skin scarred by a burn or by a sore (michvah and shechin); the bald area at the front or the back of the head (gabachas and karachas); and the parts covered with hair, the scalp and the beard (niskei adam). Outside the body altogether there are nigei begadim and nigei batim, clothing and houses that turn a strong green or a strong red. On or basar itself, three signs can bring tumah: se'ar lavan, a pair of dark hairs that have turned white; pisyon, further growth of the nega; and michyah, a spot of sound skin that shows up inside the white blotch which raised the suspicion of tzara'as to begin with. It is michyah and pisyon that this mishnah measures against each other.
Where michyah is stricter
"Yeish bemichyah mah she'ein bepisyon, veyeish bepisyon mah she'ein bemichyah." Michyah has features that spreading lacks, and spreading has features that michyah lacks.
First: "shehamichyah metamei bis'chilah," michyah conveys tumah already at the beginning. Spreading has no effect until a week of confinement has been completed and the kohen returns; michyah, by contrast, works on the spot, just as white hairs do. So if the kohen arrives at the man, or the man is brought before the kohen, and sound flesh is visible inside the white area at that very first examination, he is pronounced tamei immediately. He becomes a metzora muchlat, dispatched beyond the camp, obliged to announce "tamei, tamei," bound by every law that governs a metzora.
Second: "umitamei bechol mareh," michyah conveys tumah no matter how it looks. According to the Rav, the point is that healthy flesh differs from person to person, and any of those appearances qualifies: a man of dark complexion will have a dark spot of sound skin, a very fair man a correspondingly fair one, and a man of ruddy coloring a reddish one. The particular shade makes no difference at all, provided it is simply the natural tone of that man's own unaffected skin.
The Rambam reads the words along a different line. As the Tosfos Yom Tov quotes him from Hilchos Tumas Tzara'as, chapter 3, halachah 2: a spot that presents as red, or as black, or even as white counts as a michyah, so long as the white is not among the whites that are capable of making a nega susceptible to tumah, for a white of that intensity is by definition not the color of sound flesh. The Tosfos Yom Tov directs the reader to what he himself writes further on, in chapter 6, mishnah 6.
Third: "ve'ein bo siman taharah," michyah carries nothing within it that can produce taharah. On this score spreading is the more lenient of the two, for when the growth has covered the man from head to foot he is thereby rendered tahor. Michyah presents no such possibility. Its only function is to declare a man a tamei metzora; it is never the instrument through which he becomes tahor.
Where pisyon is stricter
"Yeish bepisyon," spreading has its own severities. "Shehapisyon metamei" "bechol shehu": the tiniest amount of growth already conveys tumah. Once the kohen has traced the outline of the nega, whatever passes that line, no matter how minute, is pisyon.
"Umitamei bechol hanega'im," spreading conveys tumah in every type of nega. The preceding chapter established that all of them are governed by pisyon: each of the four types of nigei adam, and nigei begadim and nigei batim along with them. Of all the signs discussed in this whole area, pisyon is the one that knows no exceptions and functions across the board. The same statement was made when pisyon was compared with se'ar lavan, and it appears again here as against michyah.
Spreading also conveys tumah chutz lanega, beyond the nega, which stands to reason, since growth by definition extends past the original blotch, as long as it remains attached to it. "Mah she'ein kein bemichyah." A spot of sound flesh has no standing outside the nega whatsoever. It has to lie inside the original white patch, positioned in its middle, shaped as a square, and no smaller than an adashah, a lentil. The Rosh cites from the Toras Kohanim a method of gauging that measure: thirty six hairs' breadth of area, six across and six down. In short, michyah demands both a minimum size and a fixed placement, while pisyon requires neither, taking effect in any quantity and precisely past the outline of the nega.
So the mishnah leaves the two signs evenly matched. Michyah is heavier in that it defiles from the opening examination, defiles regardless of the shade of the sound flesh, and holds out no path to taharah. Pisyon is heavier in that even a hairsbreadth is enough, that it governs every variety of nega, and that its tumah reaches beyond the borders of the nega itself.