Keilim, Chapter 19, Mishnah 10. This mishnah presents a dispute about a vessel that has been ruined for its primary purpose, and asks whether a secondary use that it can still serve is enough to keep it susceptible to tumah.
The vessel under discussion is called a mashpelet. It was a large container used for hauling manure. Its construction is important for understanding the mishnah: the opening through which the manure was loaded and emptied was on the side rather than on top. That meant the upper surface remained flat and usable, so a person could sit on the mashpelet even while it was in active service as a container. Because of that, a mashpelet was subject to two separate tracks of tumah: the ordinary tumah of a vessel, such as corpse tumah, and also tumat midras, the tumah that a zav or similar person transmits to something designated for sitting or leaning.
The Case of the Mishnah
The mishnah opens: "Mashpelet shenifchetah milekabel rimonim", a mashpelet that has been broken to the point where it can no longer hold pomegranates. In other words, holes opened in its floor large enough for a pomegranate to pass through, so pomegranates placed inside would simply fall out.
The consequence for ordinary tumah follows from a principle established several perakim earlier: a large container that develops a hole of that size is no longer regarded as a functioning vessel at all. It is considered broken, and a broken vessel cannot contract corpse tumah. On that point everyone agrees.
The Dispute
"Rabbi Meir metamei", Rabbi Meir rules that it is still susceptible to tumah, specifically to tumat midras. His reasoning is that the damage to the floor has taken away nothing from the flat top. A person can still sit on this mashpelet and use it as a bench. Since it retains a genuine function as something to sit upon, it remains a vessel for the purposes of midras and can become tamei that way.
"Vachachamim metaharin", the Chachamim rule that it is tahor. In their view it can no longer contract tumah at all, not even tumat midras. Their reason is stated in the mishnah: "mipnei shebatal ha'ikar, batlah hatefeilah", once the primary function has fallen away, the secondary function falls away with it. This is the rule already encountered earlier in the masechet. The essential identity of a mashpelet is that of a container; that is what it was made for and that is its main use. Sitting on it was never more than a secondary convenience that rode along with the primary purpose. Once the vessel can no longer contain, the primary purpose is gone, and the subordinate use of leaning or sitting on it is nullified along with it.
The Practical Outcome
According to the Chachamim, then, the broken mashpelet stands entirely outside the system of tumah: it cannot become tamei through corpse tumah, and it cannot become tamei through tumat midras either. Rabbi Meir agrees regarding corpse tumah but keeps the vessel within reach of tumat midras, because in his estimation the surface a person sits on remains a use in its own right and does not depend on the container beneath it.