Negaim, Chapter 7, Mishnah 2. The subject of this perek is white spots that never enter the framework of tumas tzora'as at all. Our Mishnah picks up the list begun in the Mishnah before it, and then pushes the inquiry a stage further.
First, a reminder of what a beharis is. We are speaking of the white shades of skin coloration, the ones that are capable of turning tomei once a siman tumah shows itself in them. There are three such simanim: pisyon, when the spot spreads; se'aros levanos, two hairs inside the spot that have turned white; and michyah, an island of healthy flesh sitting in the middle of the spot in the shape of a square.
The Rule Established in the Previous Mishnah
The earlier Mishnah gathered cases of beharos that are tohor, and the basis for all of them lies in how the pasuk is worded. The Torah writes of "odom", a person, "ki yihyeh be'or besoro": that a spot shall come to be upon the skin of his body. The verb is cast in the future, which rules out any white marking that was already present. The opening case on that list is a beharis that a person carried on him before matan Torah. That one is purely historical, mai dehavah havah, finished long ago, yet the yesod it establishes controls everything that follows, and those remaining cases are fully relevant nowadays.
Here are the cases. Be'oved kochavim venisgayer: a gentile with a beharis on his skin who then became a ger. Because the spot developed at a time when he stood outside the laws of tzora'as, it counts for nothing once he is a Yisroel and can never take effect. Bekoten venolad: an infant who comes out of his mother already marked with white spots. Inside the womb the spot was in a concealed location, and in any event the requirement of skin upon a person's body, as the pasuk demands, was not yet met, so a beharis older than his birth will never make him tomei. Bekemet venigleh: the beharis was tucked away inside a crease or fold of the body. Wherever the kohen's eye cannot reach is a beis hasesarim, and tumas tzora'as does not operate there. Later the body shifts, the crease flattens, and the spot comes into view. Even though its presence was known to us the whole time, the halachah here matches the infant in the womb: a spot that lacked the power to bring tumah at the moment it arose has no such power afterward either. The pasuk insists that from the instant it comes into being upon the skin of his flesh it be fit to become tomei.
Bero'sh uvazokon: the hair-bearing scalp and the area of the beard possess a tumah of their own, nesek, which the coming perakim will treat. That is an altogether separate category, with its own range of colors and its own siman tumah: there the hair does not whiten, it turns a yellowish, golden shade. Consequently, even white patches visible among the hairs of the head or the beard do not qualify as a beharis in the first place, and they are entirely tohor. Bishchin uvamichvah uvekedach hamordin: a michvah is damage caused by heat, whether by fire itself or by something heated. A shchin covers a wound from any other source that breaks the skin down: a blow, sickness, an infection, a fungus. A kedach seems to be a subdivision of michvah, for the Torah names only shchin and michvah; a kedach would be a burn caused by a toldah of fire, such as the waters of Chamei Tverya. The word "mordin" tells us that these sores are still open and weeping, with fluid seeping from them. A white spot that arises within such a sore falls under the tumah of shchin and michvah rather than the tumah of beharis, and therefore no tumas beharis can attach to it.
In each of these instances a white spot did exist, but it was located in a place or a condition that blocked it from ever acquiring tumah as a beharis.
When the Disqualification Falls Away
Our Mishnah proceeds to ask what the halachah is once the factor that disqualified the spot is no longer present. "Chozer haro'sh vehazokon venikrechu": a man carried beharos on his head or in his beard, and later the hair in that area dropped out. A smooth, hairless surface certainly does fall under the laws of beharos. Even so, because the marking had no ability to generate tumas beharis when it first came up, that inability stays with it, and it accomplishes nothing.
Next: "hashchin vehamichvah vehakedach" - a white spot came up inside a shchin, a michvah or a kedach, where it held no status as a beharis, and then "vena'asu tzareves": the sores began to dry, close and heal into scar tissue. At that point the tumah of shchin and michvah has departed, while the white spot remains right where it was. It too is tohor, since throughout the period that the shchin and michvah occupied that spot the marking bore no tumas beharis at all. For every one of these cases the halachah is tehorin.
Valid, Then Voided, Then Valid Once More
The Mishnah now takes up a scenario in which the marking did have force at the start, then lost that status, and afterward had the impediment removed.
The text reads "haro'sh vehazokon" - the head and the beard - "ad shelo he'elu se'or, he'elu se'or venikrechu": picture a baby born without hair, the case at the close of Chapter 6, or a lad whose beard has not yet come in. Beharos surfaced in those places. No siman tumah ever appeared in them, yet they possessed the standing of a mareh beharis and were fully able to become tomei. Then hair grew. With hair present, the region leaves the realm of beharos and passes into the realm of nesek, so the shem beharis was stripped away; a hafka'ah occurred. Later the hairs dropped out again, and the area once more comes under the laws of beharos. We are left with a beharis at the outset, a beharis afterward, and an intervening span during which it had no standing as a beharis whatsoever.
The identical pattern governs the parallel case: "hashchin vehamichvah vehakedach ad shelo na'asu tzareves, na'asu tzareves vechoyu." Here the white marking arrived first, and only later did a shchin, michvah or kedach break out in that very place while it was still open and before it scarred over. Why does such a sore void the marking's standing? Because the Torah requires that the shchin and the michvah precede the nega, as the pasuk states, "vehoyah bimkom hashchin se'eis": a se'eis that arises where a shchin had already been, which is to say afterward. In our case the white coloration came earlier, the shchin and michvah then developed and cancelled its capacity for tumah, and afterward they scarred and healed completely, with the white marking still sitting in place.
The Machlokes
On these last cases, Rabbi Eliezer ben Yaakov rules them tomei, giving as his reason "shet'chiloson vesofon tomei": both their initial state and their final state are states of tumah. When the marking first emerged it held the status of a mareh beharis able to bring tumah, and once the intervening stage was over it returned to that very status, so the interruption is of no consequence to him.
Vachachomim metaharin. The Chachomim demand that the marking retain its validity as a beharis from start to finish, unbroken. Should a hafka'ah have taken place along the way, with the very name beharis lifted off that spot, then even after the impediment is gone and the spot again sits in a location fit for tumah, it is no longer a beharis. Its standing is like that of the beharis carried by a ger: whatever it may have been in the past no longer counts for anything.