Keilim, Chapter 26, Mishnah 6. This chapter is occupied with leather articles: which of them count as a genuine keli, and which of them can pick up tum'at midras, the tum'ah that comes to an object because a person who is tamei sat, stood or lay upon it. An object receives midras only if it is made for sitting or lying, or at least is regularly used that way. Our mishnah takes several leather items used for storage and asks, item by item, whether people ever end up resting on them.
The mishnah begins with two leather articles used for clothing:
"Av kesut ve'tachrich kesut, midras."
An av kesut is a leather bag in which garments were kept, and a tachrich kesut is a leather sheet in which garments were wrapped for storage. Both of these, rules the mishnah, are subject to tum'at midras. The reason is simple and drawn from ordinary life: a soft leather bag or wrap holding clothing invites a person to sit down on it, and since that happens from time to time, the item is considered fit for sitting and can receive midras.
Purple wool: a dispute of Beit Shammai and Beit Hillel
"Av argaman ve'tachrich argaman."
About these two, the mishnah records the debate: Beit Shammai place them in the category of "midras," while Beit Hillel say they are only "tamei met."
Now the mishnah turns to the very same two articles, a leather bag and a leather wrapping sheet, when they are used not for ordinary clothing but for argaman, purple wool. Here the two schools disagree.
Beit Shammai hold that these too receive tum'at midras, and needless to say they receive tum'ah through maga, contact, as well. Even though argaman was wool of the highest grade, wildly expensive and precious, so that its owners took pains to keep it clean, Beit Shammai maintain that people nonetheless sit or lie on such a bag or wrap now and again, and that occasional use is enough to make it fit for midras.
Beit Hillel disagree. In their view these articles can become tamei met, tamei as an ordinary vessel that came into contact with a source of tum'ah, but not through midras, because they were never intended to be sat or lain upon. When a person is handling wool of that value, he does not ordinarily stretch out on the bag that holds it. Since it is not designated for lying down, midras does not apply.
A skin made into a cover
"Or she'asahu chipui la'keli, tahor."
A hide that a person prepared as a covering for a vessel is entirely tahor: it cannot receive tum'ah at all. A mere cover is not a keli. To be susceptible to tum'ah, an article must actually perform work, function as a genuine utensil, and not simply lie over something else protecting it.
"La'mishkalot, tamei."
But if he designated the hide as a covering for the weights used on a scale, it is tamei. Two reasons combine here: such a covering is formed with a receptacle, shaped like a container, and it is in fact sometimes used to hold the weights rather than merely to cover them.
Why cover weights at all? They would place the hide over the weights so that the weights would not become chipped or dented. A nicked weight is no longer exact, and once it loses its precision it can no longer be relied upon to establish that a given amount is truly the stated weight. The whole point was to keep the weights precisely as they were.
"Rabbi Yose metaher mishum aviv."
Rabbi Yose rules that the hide stays tahor, and he presents this ruling as one he received from his father. His logic rests on the article's dominant function: this piece of leather exists in order to lie over the weights, and that is how it serves virtually all the time. An object whose main role is merely to cover is not a keli, and so no form of tum'ah can take hold of it.
So our mishnah moves along a single line of thought: an article's status in tum'ah is decided by how people actually use it. Leather that holds clothing gets sat upon, and so it receives midras; leather that holds precious purple wool is the subject of a dispute between Beit Shammai and Beit Hillel; and a hide that only covers is no keli, unless its form and use make it into something more, on which Rabbi Yose in his father's name still says that the main use rules.