Keilim, Chapter 27, Mishnah 10. Our chapter has been occupied with the measurements of cloth and the different kinds of tumah that attach to different sizes. Here we meet another dispute between the Tanna Kamma and Rabbi Yose, and the subject is a cloth that carried tumat midras and then became smaller: once it has been reduced below the shiur, does any form of tumah still cling to it going forward?
The case is stated simply: "sheloshah al sheloshah shenechelak", a cloth that measured three tefachim by three tefachim, the shiur for tumat midras, and which indeed became tamei with tumat midras, and then it was torn and divided into two pieces.
The first ruling is "tahor min hamidras": each of the two pieces is now clean of tumat midras. The reason is a matter of measurement. Midras tumah rests on a cloth only when it is at least three by three tefachim, and neither half reaches that size any longer. Whatever midras the cloth once had cannot continue to sit on fragments that are too small to hold it.
But the Tanna Kamma does not release the pieces entirely: "aval tamei maga midras", each half remains tamei as a rishon letumah, on the grounds that it touched something bearing tumat midras. How so? Look at the cloth as made up of its own individual parts. Every part of that cloth was in contact with the whole cloth, and the whole cloth was tamei with tumat midras. That contact took place while the cloth was still whole and still carried midras, so even now, after the division, each piece retains the tumah of one that touched a midras object.
Rabbi Yose objects: "vechi be'eizeh midras naga zeh?" Which midras, exactly, did this cloth touch? Nothing outside itself ever touched it. All it ever touched was its own self, and self-contact of that kind is a concealed, internal contact, not genuine contact at all. And at this moment there is no midras anywhere in the picture, since both pieces are below the shiur for midras. The result is that the torn cloth stands before us like an entirely new creation: it has lost its midras, and it has lost the maga as well.
For Rabbi Yose, then, both fragments are fully tahor, and the mishnah goes on to spell out the single way tumah could still reach them: "ella im kein", unless, that is, "naga bo hazav", a zav (or any other av hatumah) comes along and touches the piece after the tearing has already happened. In such a case the cloth is certainly tamei, yet the mishnah is precise about the category: "tamei maga hazav", its tumah is that of an item that came into contact with a zav, generated by a fresh contact occurring now, and not any remnant of the status it bore back when it was still whole.
The dispute, then, turns on a single question: can an object's own body be counted as having touched itself? The Tanna Kamma is willing to view the cloth as a collection of parts in contact with a midras whole, while Rabbi Yose insists that contact means contact with something else, and that a cloth reduced below its shiur begins its life again from the start.