Ohalos, Chapter 13, Mishnah 6. This chapter deals with windows and other gaps in a wall, and with the measure of one tefach square. Tumah lying in one room travels through an opening that measures a tefach in each direction into the room beyond it. Should something be set into that gap so that a full tefach square no longer stands open, the tumah is held back. The question, then, is which items may be relied upon to cut down the opening and which may not. The mishnah before ours listed the things that do cut it down; ours turns the coin over: eilu she'einan mema'atin, these are the items that do not reduce the measure. The meforshim note that this list is not merely the earlier one in reverse: every clause carries its own chidush.
A bone and a piece of flesh
"Ein ha'etzem mema'et", a bone does not reduce the opening, "al yedei atzamos", when it stands opposite bones. A bone smaller than a se'orah bears no tumah on its own, yet it will not narrow the window if what lies tamei inside is a heap of bones. Rather than acting as a partition, the small bone attaches itself to the bones within and turns into part of the tumah.
"Velo basar al yedei basar." Flesh behaves the same way. A piece of a meis's flesh under a kezayis is not metamei by itself, yet it will not bring the gap below the required measure when what fills the room is flesh of a meis. It joins the rest and becomes one with it.
"Velo kezayis min hameis, velo kezayis min haneveilah." Now the item wedged into the window is itself tamei: a kezayis of a meis's flesh, or a kezayis of neveilah. Each is a tumah in its own right, and something tamei can never form a chatzitzah against tumah, so the window is not reduced.
"Velo ka'adashah min hasheretz." Likewise a lentil's worth of a dead sheretz, the measure at which a sheretz is metamei. It too is a tumah, and a tumah cannot stop tumah.
Food set into the gap
"Velo kebeitzah ochlin." A kebeitzah is the amount at which food contracts tumah on the Torah level, and also the amount at which such food conveys tumah onward. Since it is a candidate for tumah, it is unfit to narrow the opening. Most meforshim understand the mishnah to be speaking of food already huchshar, wetted by one of the seven liquids; short of that, the food could not receive tumah in the first place.
"Velo tevuah shebachalonos." Grain whose roots have taken hold inside the window does not reduce it. In the earlier mishnah, produce rooted at a distance from the wall sends its branches over and into the gap, and such produce may well be left standing. Rooted grain inside a window, though, no one tolerates, for roots in a wall wreck the wall, and the owner is certain to tear it out. Here is a yesod worth filing away: whatever a person has no intention of leaving in place does not block tumah.
"Velo chochai she'ein bo mamash." Spider webs come in more than one variety: some are dense enough to hold together, others so fine that a touch scatters them. Our mishnah refers to the fragile sort. They possess no chashivus, and one cannot call them a sealing of the window.
Neveilah of a bird
"Velo nivlas ha'of hatahor shechishav alav." "Tahor" in this clause has nothing to do with tumah and taharah; it denotes a bird permitted for eating. Once a person had it in mind to eat this neveilah, it acquires the status of ochel and takes on tumas ochlin, and being tamei, it can no longer stand against tumah.
"Velo nivlas of tamei shechishav alav vehuchsherah." Here too, "tamei" describes a bird forbidden for eating. Such a neveilah loses its power to narrow the window only when two things are true: a person thought of eating it, and it also underwent hechsher, contact with a mashkeh of the shiva mashkin. (How many people's estimation is needed to turn something into food, and what that thought involves, we will meet later on, b'ezras Hashem yisbarach.)
Threads struck with a nega, and a brick of plowed-field soil
"Velo hashasi veha'eirev haminuga'im." Shasi are the warp threads, running lengthwise and generally the thinner ones; eirev are the woof threads, running crosswise and somewhat thicker. Haminuga'im: these spun threads, still awaiting the loom, were afflicted with tumas tzara'as, a topic we will study at length in Maseches Nega'im. Since a nega has left them tamei, they do not reduce the measure, even though they physically fill the window and no full tefach square remains open. A tamei item is simply incapable of the job. Worth noting in passing: a nega is essentially the only route by which threads become tamei.
"Velo leveinah mibeis hapras, divrei Rebbi Meir; vachachamim omrim: haleveinah mema'etes, mipnei she'afarah tahor." What is a beis hapras? A field that once held a kever and was afterward plowed, so that fragments may have been strewn about in it. On a Torah level nothing need worry us, since we assume the bones were pulverized and nothing as large as a se'orah survived, and a fragment beneath that size conveys no tumah. Chazal nevertheless decreed that soil of such a field is metamei by touch and by carrying, lest a bone of a kese'orah be lying there. Bricks were manufactured out of earth, and here someone worked up his mixture and shaped a brick out of soil taken from that plowed field. Rebbi Meir holds that the rabbinic decree extends even to loose soil and is not limited to a gush, a solid clod; the brick is therefore tamei and cannot stop the tumah. The Chachamim maintain that the decree was placed on a gush alone, a compacted lump, and not on crumbled dust. A brick formed from that dust is accordingly tahor and does reduce the opening, "mipnei she'afarah tahor".
The klal
"Zeh haklal: hatahor mema'et, vehatamei eino mema'et." Anything tahor, once it sits in the window in such a way that genuinely less than a tefach square is left open, cuts down the measure, and the tumah stays where it is instead of crossing from house to house. Anything tamei does not, and that covers both an item that is a tumah itself and an item liable to receive tumah.
One question deserves revisiting. Why is thought alone enough to disqualify the neveilah of a permitted bird, with no hechsher demanded, while the neveilah of a forbidden bird requires both thought and contact with a liquid? Because the neveilah of a kosher species holds a latent power of tumah of its own: touching it does not defile, but a person who swallows it becomes tamei with a severe tumah, to such a degree that a keli in his hand at that moment turns tamei. We will see more of this later in Mishnayos, b'ezras Hashem yisbarach. The principle is that any ochel capable of generating such a tumah chamurah becomes tamei even with no hechsher, even if no liquid ever reached it. The neveilah of a non-kosher bird bears no tumas neveilah at all, and so it needs both the thought that turns it into food and the hechsher that opens it to tumah.