Keilim, Chapter 22, Mishnah 6. Our Mishnah continues the discussion of a wooden chair whose seating surface is made of separate wooden slats laid across the frame. The question throughout is a simple one with practical consequences: once some of those slats are missing, is the object still a chair? If it can still serve a person as a seat, it remains a utensil and can become tamei; if it can no longer be sat upon, it has lost its status as a kli.
Picture the chair under discussion: three wooden slats resting across the top, side by side, forming the seat. The Mishnah presents two cases.
Take the first situation: the central board has been taken out, while the two boards along the edges stay where they were. The Mishnah puts it this way: "Kise shenital chipuyo haemtza'i", a chair whose middle slat was removed, "vehachitzonim kayamim", its outer slats still in place, is "tamei". A gap now runs down the length of the seat, yet a person can lower himself onto the two boards that are left, and so the object goes right on doing the work of a chair.
An important detail is assumed here. We are speaking of a chair whose side panels rise above the level of the seat, so that the slats sit within a raised frame. That detail matters, because if nothing protruded above the seating surface, one could simply turn the chair upside down and sit on it that way, and it would remain a usable seat even without any of the slats at all. The Mishnah's discussion of which slats remain only makes sense in a chair whose sides project upward, leaving the slats themselves as the only surface a person can sit upon.
The second case: "Nitelu hachitzonim vehaemtza'i kayam, tamei." Here the reverse happened: the two outer slats were taken away and only the middle one is left. This chair, too, remains susceptible to tumah, because a person is able to sit on that single central board, and the object continues to serve him as a seat.
Ribbi Shimon does not accept this second ruling as it stands. He sets a condition, "im haya rachav tefach": the surviving central board must measure a tefach across. To his mind a tefach is the smallest width that qualifies as something a person seats himself upon, and anything thinner does not count as a seat at all.
The Tanna Kamma, the unnamed voice with which the Mishnah began, rules otherwise. Even a central board narrower than a tefach leaves the chair susceptible to tumah, for a person can brace himself, hold his balance on the thin plank, and seat himself there all the same. Sitting has not become impossible, and so the object has not stopped being a kli.
The principle running through the whole Mishnah is therefore consistent: susceptibility to tumah follows usefulness. As long as what remains of the chair can still hold a person, whether it is two slats with a gap between them or one narrow board in the center, the chair has not been reduced to broken wood. It is still a kli, and it can still become tamei.