Negaim, Chapter 8, Mishnah 7. This chapter is devoted to pricha, the case in which the white of a nega spreads until it covers the whole body. The rule to hold on to throughout is this: when the whiteness that overruns the entire body emanates from a baheres that had already been declared muchlat, that is, a nega already ruled tamei, the pricha is metaher; but when the spreading comes from a baheres that stood in a state of taharah, the pricha is metamei. Our mishnah applies that principle to a man who carries two separate baharos, and then turns to several other details that determine whether a nega is reckoned as one nega at all.
Two Baharos, One Tamei and One Tahor
The mishnah begins: "shtei baharos, achas tmei'ah ve'achas tehorah" - a person has two white spots on him, one of which had already been ruled tamei by the kohen and one of which stood tahor. "Parchu mi'zu le'zu, ve'achar kach parchah be'chulo, tahor." The whiteness of the one spread into the other so that the two became a single nega, and only afterwards did the whiteness cover his entire body. In such a case the man is tahor.
Does it matter which of the two spread into which? The Rav explains that it makes no difference, "bein mitehorah litmei'ah" and the reverse, because once they merge, the tehorah is absorbed into the tmei'ah and takes on its status. Consequently, when the nega afterwards overruns the whole body, it is a pricha issuing from a nega tamei, which is metaher. The Rosh writes the same, and so learns the Tiferes Yisroel. The Toras Kohanim derives this ruling from the doubled expression of the pasuk, "paroach tifrach."
The Maharam MiRothenburg: The Order of Events Matters
The Tosfos Yom Tov cites a comment of the Maharam MiRothenburg here (it does not appear in every printed collection of the Maharam's notes to the mishnayos). The Maharam reads the mishnah with great precision: the tanna deliberately set the events in this sequence, first "parchah mi'zu le'zu" and only afterwards "parchah be'chulo."
The point is as follows. Had the tahor baheres first sent out its whiteness in the opposite direction, outward and away from the tamei baheres, that outward growth would itself carry the status of tamei, since it is spreading from a nega that is tahor, and there would be no clean pricha of taharah afterwards. But in the case of our mishnah, where the two spots first ran into each other, even if it was the tahor one that spread into the tamei one, they merged and became a single nega before the whiteness covered the body. Therefore, when the covering of the body took place, it counted as issuing from the tamei nega, the tehorah having become batel to the tmei'ah, and the man is tahor.
Half a Gris on Each Lip
The mishnah continues with a different question entirely: when do two small patches of white combine into a nega of the required size? "Bisfaso ha'elyonah, bisfaso hatachtonah": there is half a gris of baheres on the upper lip and half a gris on the lower lip. When the man closes his mouth, the two halves press together and to the eye they look like a full gris in a single place. The same is discussed "bishtei etzba'os, bishtei risei einav", on two fingers, or on the upper and lower eyelid.
The ruling is: "af al pi shemudbakin zeh lazeh vehein nir'im ke'echad, tahor." Even though the two surfaces touch one another and to the observer they present the appearance of one continuous gris, there is no tzeiruf, no joining. Each patch remains a half gris on its own limb, and what the eye perceives as a single nega is only an optical impression, not a halachic reality.
Bohak Holds Back the Pricha
Next: "parchah be'chulo aval lo vabohak" - the nega spread over his entire body, but it did not cover a patch of bohak on him. Bohak, as the Rav defines it, is "mar'eh lavnunis lematah me'arba mar'os", a whitish shade dimmer than the four shades of white listed in the opening mishnah of the masechta, dimmer even than krum habeitzah. Since bohak is not one of the mar'os of nega, it interrupts the covering, and therefore "ma'akeiv es hapricha." The man is tamei, because his body was not truly covered.
This is worth noting in connection with an earlier question, whether bohak might be treated as michyah. From our mishnah it appears clearly that the two are separate categories: michyah is the man's own healthy flesh showing through the white, while bohak is a patch of faint whiteness. They are not the same thing.
Bohak That Appears Afterwards
The next clause deals with bohak that surfaces only after the fact. The whiteness had already spread over the man's entire body, and then the tips of the limbs (the rashei evarim enumerated earlier, the places that are not metamei bemichyah) reemerged from beneath the covering of white, showing the faint shade of bohak, "kemin bohak". The verdict is taharah. The Rav notes that this is not a leniency peculiar to the extremities: the identical ruling governs the rest of the body as well. Bohak obstructs a covering only when it is present from the beginning; once the pricha has been fully accomplished, its later appearance takes nothing away. Why, then, did the tanna single out the tips of the limbs? Only because he will need them for the closing clause, where they serve to sharpen Rebbi Meir's position to its fullest extent.
The Dispute of Rebbi Meir and the Chachamim
"Chazru bo rashei evarim pachos mikadashah": after the covering of the body, the tips of the limbs reappeared as bohak in an area smaller than a lentil. Rebbi Meir declares him tamei. The Chachamim answer: "bohak pachos mikadashah siman tumah bitchilah ve'ein siman tumah basof." Such a patch of bohak prevents the pricha when it is present from the start, but it carries no consequence once the pricha has already taken effect.
The Rav spells out the sharpness of Rebbi Meir's view: even at the rashei evarim, which are not metamei bemichyah, and even where the exposed area is less than a lentil, once it appears as bohak Rebbi Meir declares him tamei, and all the more so on the rest of the body. Conversely, the earlier clause of the mishnah, that when the rashei evarim reappear as bohak he is tahor, follows the Rabbanan and not Rebbi Meir, as the Toras Kohanim indicates. And the size mentioned is not exact: the Rabbanan were only speaking in the terms Rebbi Meir had used, but in truth they declare him tahor even when the bohak is considerably larger than a lentil, "de'afilu tuva nami tahor", since bohak at the end does not hold anything back according to all opinions.
The halacha follows the Chachamim: bohak appearing after the pricha has already covered the body does not render the man tamei, even if it is far larger than an adashah.