Kelim, Chapter Twenty-Two, Mishnah Nine
1) A [wooden] block which was painted red or saffron, or was polished:
a) Rabbi Akiva says that it is susceptible to uncleanness,
b) But the sages say that it remains clean unless [a seat] was carved out.
2) A small basket or a big one that was filled with straw or other soft material remains clean [even] if it was prepared as a seat.
a) But if it was plaited over with reed-grass or with a cord it becomes susceptible to uncleanness.
Section one: The issue at hand in this section is what does one have to do to a plain wooden block in order to turn it into a chair? According to Rabbi Akiva, it is sufficient to paint it or to polish it for it to be considered a chair and therefore susceptible to impurity. The other sages rule that this is insufficient. To make something into a real chair a seat must be carved out.
Section two: Even if one fills a basket with soft material and intends to use it as a seat, it is not susceptible to impurity. The problem with it is that as soon as he turns the basket over, the stuff will fall out and the seat will no longer exist. To turn the basket into a seat he must fill it with some sort of stuffing and plait over it with some type of binding material to keep the contents in. If he does this, the basket will be a seat and it will be susceptible to impurity.