Keilim, Chapter 22, Mishnah 9. The previous mishnah closed with the cushion of a stonecutter, and our mishnah continues along the same line of inquiry: which improvised seats have reached the stage of a finished keli, so that they are subject to tum'at midras, the tumah that a person who is tamei transmits to something made for sitting or lying upon by pressing his weight on it? The mishnah brings two cases, and in each one the question is the same: has the owner actually completed the object as a seat, or is it still raw material that happens to be sittable?
The block of wood
The first case is a "kofet", a thick block of wood. Its owner treated it in three ways. "Sheserako", he coated it with red pigment, "vekhirkemo", and he colored it with orange, these being the two ordinary paints in use in those days, "vesacho panim", and he planed and smoothed its upper surface with tools so that a person could sit on it comfortably.
Rebbi Akiva holds "metamei": the block is now a completed utensil and can receive tum'at midras. In his view, once it has been painted and its top has been smoothed for sitting, it functions as a seat, and function is what determines the matter.
The Chachamim rule "metaharin ad sheyachok bo": the block stays tahor until its owner carves a depression into it. Before that final act of shaping, no real place of sitting has been formed, and what stands before him is still only a painted log rather than a chair, and a log does not receive tum'at midras. This is the exact point of the argument: Rebbi Akiva maintains that even now the object already does the job of something one sits upon, which is enough to render it susceptible to tumah.
The stuffed baskets
The second case: "hasal vehakalkalah shemil'an teven o mochin", a wicker basket and a reed basket that were packed with straw or with soft stuffing such as feathers, "hitkinan liyeshivah", and the owner then designated them for sitting, so that the filling turns them into cushions. One might assume that a stuffed basket is a cushion in every respect. The mishnah rules "tehorin", they are tahor and cannot contract tum'at midras. The reason is that they have not been closed up. Their mouths are open, the stuffing can spill out with no effort at all, and an object whose filling is liable to escape is not yet a proper seat. Lacking that, it is not susceptible to tumah.
But "seragan begomi o bimshichah teme'in": if he laced the mouth of the basket shut with reed grass or with cord, so that the stuffing is secured inside and will not fall out, then they are teme'in, capable of receiving tum'at midras. Once they are bound and finished in this way, they are genuine cushions meant for sitting, and their status changes accordingly.
The thread running through both halves of the mishnah is the same. Tumah attaches to a keli, and a keli is something whose making has been brought to completion. For the Chachamim in the first case that means a carved seat, for Rebbi Akiva it means a surface already fit to sit on, and in the second case it means a mouth tied closed so the cushion holds together. Until then, we are looking not at furniture but at the materials from which furniture will eventually be made.