Keilim, Chapter 22, Mishnah 7. This chapter deals with wooden furniture: tables, benches and chairs, and with the question of what happens to their tumah status once they are damaged. A chair, so long as it can be sat upon, is subject to tumat midras, the tumah that a zav or a nidah transmits by sitting or leaning on an object made for sitting. Once the chair is broken to the point that it can no longer be used for sitting, that avenue of tumah closes. Our Mishnah discusses the wooden chair that has been under discussion, this one having a storage compartment built into the seat itself. Since any wooden keli with a receptacle is in principle able to receive tumah, the question arises: even after the chair is ruined as a chair, does the receptacle in the seat keep it in the category of a keli that becomes tamei?
The case opens with the words "Kisei shenitlu shenayim mechipuyav": a chair from which two of the boards that make up its seating surface were taken away. The Mishnah then adds "zeh besad zeh", telling us that the two missing boards had lain right next to each other, so what remains is one continuous opening rather than two small separate gaps. Nobody can settle himself securely on a seat like that. And as in the other rulings of this chapter, picture a chair whose side frames rise higher than the seating surface; were that not the case, one could simply invert it and sit on the base quite comfortably, so it would never have forfeited its identity as an item meant for sitting. Here the sitting surface itself has been broken through, and the piece can no longer do a chair's job.
Now comes the dispute: "Rebbi Akiva metamei, vaChachamim metaharin." Rebbi Akiva holds that the object still receives tumah. True, it is finished as a seat, so midras no longer applies to it. But it still has a receptacle built into the seat, and a wooden vessel that holds something is a keli that becomes tamei. The compartment gives the object independent standing as a keli, and therefore it can contract ordinary tumah through contact.
The Chachamim disagree and rule that it is completely tahor. Their reasoning is that we must look at what this object was made to be. Its purpose was to serve as a chair, not as a container. The storage space in the seat was never more than an added convenience alongside the chair's real function. Once the chair can no longer be sat upon, the object has lost the very thing that made it a keli, and the leftover compartment is not enough to sustain its status. From this point onward it is pure.
Rebbi Yehudah now stretches the ruling of the Chachamim to a case that lies one step further along. In his words, the law is the same regarding "af kisei shel kalah", even a chair set aside for a bride at her wedding, "shenitlu chipuyav venishtayer bo beit kabbalah, tahor": its covering boards were taken off, a hollow capable of holding things was left behind in it, and it is nevertheless pure. Tumat midras no longer attaches to such a chair, and it does not pick up tumah through contact either.
The Mishnah supplies the principle behind this: "mipenei shebatal ha'ikar uvatlah hatefeilah", because once the primary function has ceased, the secondary function ceases along with it. The compartment is the tefeilah, the subordinate element; it lives entirely off the object's main identity. When the main identity is gone, the subordinate one has nothing to stand on and it, too, is nullified.
Rebbi Yehudah's chiddush lies precisely in the bridal chair. Such a chair, even with slats missing, might still be serviceable enough that ordinary people could sit on it. Nevertheless, since it was designated for a specific role, to be the bride's chair at the wedding, and it can no longer fill that role with dignity, its defining function has been lost. Its own purpose has ended, and therefore, despite the receptacle built into the seat, it cannot become tamei.