Keilim, Chapter 20, Mishnah 4. Earlier in this chapter we learned about a kneading bowl (areivah) that developed a crack and could no longer serve its original purpose, so its owner began sitting on it instead. There we were taught that a bowl of a certain capacity, one that holds between two log and nine kav, is subject to tum'at midras, the impurity transmitted by the pressure of a person who is himself impure. Our mishnah takes up the next case: a kneading bowl whose capacity is larger than nine kav. Can such a vessel, once it has broken, be turned into a seat and become tamei?
The mishnah opens: "areivah gedolah shenifchetah mikelabel rimonim", a large kneading bowl, one holding more than nine kav, that became damaged to the point that it can no longer hold pomegranates. The hole in it is wide enough that pomegranates would drop straight through. A vessel damaged to that degree is no longer a functioning keli at all, and a vessel that has lost its status as a keli cannot receive tum'ah.
The mishnah continues: "vehischinah liyeshivah", the owner prepared it for sitting. He did nothing physical to the bowl; he simply decided that from now on this is what he will sit on. "Rabbi Akiva metamei", Rabbi Akiva rules that it can now contract tum'at midras. Since it is in fact being used as a seat, that category of tum'ah applies to it.
The principle behind Rabbi Akiva's view is that kelim can acquire a new identity through kavanah, through a person's intention alone. An object that broke and thereby lost its ability to become tamei can be restored to the world of tum'ah by nothing more than a thought, a new purpose assigned to it. Our kneading bowl was beyond use and beyond tum'ah, yet the owner redirected it: this will be a seat. For Rabbi Akiva, that decision is enough to make it capable of becoming tamei once again.
The mishnah then records the dissenting opinion: "vachachamim metaharin ad sheyekatze'a", the Chachamim hold that the bowl remains tahor until the owner actually cuts it and shapes it so that it is properly fit for sitting. In their view, intention by itself does not convert a kneading bowl into a seat. A physical change is required. The owner must do something to the vessel itself, and only then does it become capable of receiving tum'ah again.
The mishnah now presents a different use for the same damaged bowl: "asa'ah evus labehemah", he made it into a feeding trough for an animal, a place from which the animal eats. In that case the vessel can become tamei through contact with a source of tum'ah, like any ordinary container. True, the large hole in it disqualified it as a kneading bowl, but it is entirely serviceable as a trough. Once it has been turned into a trough, it is a functioning keli again and tum'ah takes hold of it.
One more ruling closes the mishnah: "af al pi shekeva'ah bakotel, temei'ah", even should he set the trough firmly into the wall, it can still contract tum'ah. As a rule, whatever is mechubar lakarka, fastened to the ground, stands outside the laws of tum'ah entirely. That exemption does not help here, since the trough already carried the status of a keli at the moment it was installed. Where an item was a genuine vessel first and only afterward became fixed in place, its susceptibility to tum'ah stays with it.