Negaim, Chapter 11, Mishnah 7. Our chapter is devoted to nigei begadim, the tzoraas that strikes clothing, and it pays to look first at the pesukim in Tazria that treat this subject, because everything the Mishnah teaches here rests directly on the words of the Torah Shebichsav. Our mishnah joins the process at a later stage: the nega on the garment had grown faint, the spot was torn away and burned, and a matlis, a patch, was then sewn into the hole. Why sew in a patch at all? So that the same spot should still be able to become tamei again, for the Torah speaks of a nega that appears "oad," once more, in the pasuk "ve'im teiroeh oad babeged."
Chozer labeged: the patch is saved
"Chozer nega labeged, matzil es hamatlis." The patch is in place, and the nega returns, but not to the patch. It reappears on a different part of the garment. Even though the patch was inserted precisely in order to fulfill "oad," here the nega did not come to the patch. The garment must be burned, but before it is burned the patch is taken out and saved. It is not destroyed with the garment; it now requires hesger of its own.
Chozer lamatlis: everything burns
"Chozer lamatlis, soreif es habeged." If instead the nega returns to the very place that had been torn out, that is, to where the patch now sits, then the entire garment together with the patch is burned.
Hatoleh min hamusgar betohor
The mishnah now takes up a different scenario. As the Mishnah Achronah explains, what happens here is done against the din: the garment is supposed to remain in hesger, and instead someone comes along, cuts a piece out of it and sews that piece as a patch onto a clean garment, a beged that is tohor. This is "hatoleh min hamusgar betohor," one who patches from a garment in hesger onto a tohor garment.
"Chozer nega labeged, soreif es hamatlis." If the nega returns to the original garment, in some other spot on it, then the patch is burned as well. Although that piece is now sewn onto a garment that is tohor, it is still regarded as belonging to the original garment. So the garment burns, and the piece that was removed from it and used as a patch elsewhere must be taken off that second garment and burned along with it.
Chozer lamatlis: the first garment burns, the second goes into hesger
"Chozer lamatlis, habeged horishon yisoreif." Now the nega breaks out on the swatch itself, which has already been sewn onto the tohor garment. The ruling: the beged it was taken from is burned. That is remarkable. We never stopped counting that swatch as a limb of its parent garment. Had the swatch never been detached, a nega surfacing on it would obviously have meant that tumah had come back to that beged; and so even now, when the piece sits somewhere else entirely, we judge the matter as though it had never moved, and the original beged goes to the fire.
"Vehamatlis teshameish es habeged hasheini besimonim." Simultaneously, this same swatch, which came from a beged now condemned to burning, functions for the second garment too. The Rav explains the word besimonim to mean that the second garment enters hesger along with the patch: from here on, the second beged is judged exactly as if the nega had erupted on it, both for the purposes of hesger and for the purposes of sreifah. One nega, then, works in two directions at once: a stringency looking backward, since the original beged is burned because of it, and a fresh status looking forward, since the second beged is now under observation.
The Rav pushes this one step further. Suppose the nega vanishes from the swatch altogether, and only later does a nega surface somewhere else on the second garment. That later nega already carries the consequence of burning, because we view it as a continuation of the nega that had appeared on the patch. His reasoning: the moment the nega came back onto that swatch, the garment it was attached to was already earmarked for the fire. Hence even once the swatch itself is clear, a subsequent nega anywhere else on that beged means the whole of it must be burned.
This is a demanding mishnah, and its principle is worth holding onto: a piece cut from a garment in hesger never quite leaves the garment it came from, and yet it also draws its new host garment into the din. A nega on such a patch reaches backwards to burn the first beged and forwards to place the second beged under watch.