Keilim, Chapter 27, Mishnah 9. This chapter deals with cloth items and the ways they contract tum'ah, and our mishnah returns to a theme that has already come up earlier in the chapter: what happens to a garment or sheet that carried tumat midras and is then converted into something else.
By way of background: when a zav lies or sits on an item that is designated for lying or sitting, that item becomes an av hatum'ah through tumat midras. Earlier in the chapter we met the principle that such an item can also be regarded as having touched something bearing tumat midras, namely itself, and so beyond its status as an av hatum'ah it also carries the lighter status of a rishon le'tum'ah. That is the very distinction that produces the debate in front of us now.
The case of the mishnah
"Sadin shehu tamei midras ve'asa'o vilon": a sheet became tamei with tumat midras, say a zav lay upon it, and its owner then turned it into a curtain, a window shade. "Tahor min hamidras": as far as tumat midras is concerned it is now pure, since a curtain is not an item made for lying or sitting upon, and only such items are subject to midras. "Aval tamei maga midras": nevertheless it retains the lesser status of a rishon, on the grounds that while it was still a sheet bearing tumat midras it was in contact with something possessing tumat midras, namely itself. That contact, the mishnah's first opinion holds, leaves behind a residue of tum'ah even after the change of use, and so the curtain is a rishon.
Rabbi Yose's objection
"Amar Rabbi Yose, vechi be'eizeh midras naga zeh": Rabbi Yose asks, and what midras did this item ever touch? In truth the sheet never came into contact with any outside object carrying tumat midras. To count its contact with itself as real contact, and on that basis to keep it tamei as a rishon even now that it has become a curtain, is in his view unfounded.
Rabbi Yose's reasoning rests on how we view the transformation. The cloth had been fashioned into a sadin, a keli of the kind that is capable of receiving tumat midras. Once it was refashioned into a vilon, into a window shade, it is a different item altogether, and it has no continuity with what it was before. The comparison is to a keli that became tamei and then broke: the breakage strips away its tum'ah entirely. So too here. The change of function does not merely remove the tumat midras; it removes the accompanying tumat maga as well, and the curtain emerges completely tahor.
How the curtain can still become tamei
The mishnah then points to the one avenue that stays open. "Ella im kein": unless, that is, a zav has now laid a hand on it ("naga bo hazav"), and it is then rendered tamei by that fresh touch ("tamei bemaga hazav"). Whatever av hatum'ah makes contact with the item in its present form defiles it, for a vilon is a keli in every respect and stands open to tum'ah from any source that reaches it from here on.
The upshot, then, is a clean line. In its new form the item is a keli like any other, exposed to any tum'ah it may encounter from here on. But its past is closed: neither the tumat midras it once bore nor the maga that came along with that midras when it was in contact with itself carries over into what it has become.