Keilim, Chapter 27, Mishnah 7. Throughout this chapter the Mishnah has been fixing the sizes a piece of fabric must reach before tum'ah can take hold of it, and one of the chapter's governing measurements is that tum'at midras applies only to a cloth of at least three tefachim by three tefachim. Our Mishnah now presses that rule a stage further. Suppose a cloth already carried tum'at midras, and only afterwards was it cut down so that it no longer holds the required shiur. Has its tum'ah disappeared altogether, or does some trace of it survive?
Before entering the cases, it is worth being precise about how tum'at midras operates. When a zav rests his weight on a chair or on a garment, that object does not receive a diluted, lower grade of tum'ah from him. It does not descend a level the way tum'ah usually descends when it is transmitted. Instead the object itself becomes an av hatum'ah, standing on the very same level as the zav. And like any av hatum'ah, whatever touches it does go down a level: a person who touches it becomes a rishon le'tum'ah, and a vessel that touches it likewise becomes a rishon. Rishon is one rung below av hatum'ah. Keep both of those points in mind, because the Mishnah's two rulings turn on the difference between being a midras and merely having touched a midras.
The first case: the cloth is completed, and only then a thread is removed
The first case: the cloth is completed, and only then a thread is removed
The Mishnah's first case begins: "Habeged she'arag bo shelosha al shelosha ve'nitma midras", a cloth in which the weaver had produced a square of three tefachim by three tefachim, exactly the shiur, and a zav rested his weight upon it, giving it the standing of a midras. He then returned to the loom: "ve'hishlim alav et kol habeged" means that he wove the garment out to its intended full size. Because the fresh weaving is bound together with the square that preceded it, midras status now covers the finished item as a whole, and is not limited to the patch that was there when the zav leaned on it.
Then something is undone. The Mishnah continues that later on, "natal chut echad mitechilato": he pulled out a single thread from the portion woven first. With that one thread missing, the earlier square falls short of three tefachim by three tefachim, and the halachah is "tahor min hamidras", the entire cloth is released from midras tum'ah.
The logic is straightforward once we remember where the tum'ah was anchored. The original square was exactly the minimum size, and the moment even a small part of it is detached it falls below that minimum and cannot hold tum'at midras. The later addition never had midras status of its own; it was tameh only by virtue of being attached to that original three by three. Once the anchor is disqualified, the additional weaving has nothing to lean on, and so the whole cloth is released from tum'at midras.
Still, the Mishnah attaches a second half to the ruling: "aval tameh maga midras". The cloth has not turned fully tahor; it holds the level of rishon le'tum'ah, earned through its contact with a midras. While the first square still bore midras status, the two stretches of fabric lay against one another, and that touching cannot be undone after the fact. A shrunken cloth can no longer serve as a midras, but whatever came into contact with an av hatum'ah walks away as a rishon. The garment in its entirety, then, is still tameh, simply one rung further down than before.
The second case: the thread is removed first, and only then the cloth is completed
The second case: the thread is removed first, and only then the cloth is completed
The Mishnah now flips the chronology. In the opening case the order ran like this: the square acquired midras tum'ah, the weaving was brought to completion, and the thread was withdrawn at the end. Here the thread comes out earlier. The weaver made his square of three by three tefachim, a zav leaned upon it and it took on midras status, and then, in the language of the Mishnah, "natal chut echad mitechilato": a thread was drawn out of that first square, which at once dropped below the shiur and forfeited its standing as a midras. Only later did he sit down and finish the piece, "hishlim alav et kol habeged". The verdict: "tameh maga midras", the garment carries the tum'ah of a rishon, as an item that came in contact with a midras.
To follow this, think about what happened inside that original three by three square itself. Even after it lost a thread and can no longer be a midras, every one of the threads within it had been in contact with something that carried tum'at midras, namely the square of which they were part. So they retain the status of a rishon through maga midras, contact with a midras.
What about the portion woven afterwards? It never touched a midras at all, since the cloth had already been diminished before this new material was joined to it. But it is being attached to a section that is a rishon, and by joining a rishon it becomes part of that rishon. The result is that the entire garment, original section and later addition together, is tameh as a rishon le'tum'ah and no more than that.
The Mishnah has thus taught us a fine but consistent principle. Tum'at midras depends absolutely on the minimum measurement of three by three tefachim: remove even a single thread from a cloth that is exactly that size, and the midras status collapses, taking with it whatever was attached to it. What does not collapse is the record of contact. Whatever touched the cloth while it was still a midras remains a rishon le'tum'ah, and whatever is later joined to that rishon shares its status. Either way the garment stays tameh; the only question the Mishnah is answering is on which rung of tum'ah it stands.