Keilim, Chapter 23, Mishnah 1. This mishnah looks at leather vessels that have something packed inside them, and asks a simple but sharp question: when the leather vessel is tamei, does the tumah reach the stuffing as well? The answer turns on one detail: is the filling merely sitting inside the vessel, or has it been sewn together with it into a single item?
The Mishnah opens with a list of such vessels: "hakadur veha'imum". The kadur is a leather ball that has been filled with rags or with hair; the leather forms the outer casing and the soft material inside gives it its shape and bounce. The imum is a shoemaker's form, a stuffed leather mold in the shape of a foot. The craftsman would build the shoe around it, so that the finished shoe would take the shape of the mold. In both cases we have a leather container holding scraps of cloth or hair as filling.
The list continues: "vehakamei'a vehatefillin shenikre'u". A kamei'a is an amulet, a small leather pouch containing a written scroll believed to have the power to heal a person or to help him. Here too the leather serves as a case for what is placed inside it. And tefillin, of course, are leather batim, boxes, with parchment scrolls set within them. Concerning all of these, the Mishnah describes a case where the vessel first became tamei and afterwards tore, so that what was inside is now open and exposed to the touch.
The tear being spoken about is a small one. It is not the kind of damage that ruins the vessel and leaves it with no further use; were that the case, the vessel would lose its status as a kli altogether and would no longer be able to carry tumah. Since it is only slightly torn, it remains a vessel and retains its tumah.
The ruling: "hanogei'a bahen tamei". One who touches them, meaning one who touches the leather casing itself, becomes tamei, since he has touched a leather vessel that is tamei. The Mishnah is speaking of leather vessels that contracted tumah from a corpse, which gives them the severe status of an av hatumah, and in that status they are able to transmit tumah to a person who touches them.
"Uvemah shebetochan tahor". But one who touches what is inside them remains tahor. The filling, the rags, the hair, the scroll, was never joined to the outer casing. It simply rests within the vessel, held in place by it, and nothing more. Since it is a separate item and not part of the tamei vessel, touching it does not convey tumah.
The closing clause of the Mishnah draws the contrast. It turns to an ukaf, a leather saddle laid across an animal's back, which is likewise packed with padding: "ha'ukaf shenikra", a saddle that split open after it had already become tamei, so that its filling lies exposed. Here the ruling is "hanogei'a bemah shebetocho tamei": whoever handles the padding within is rendered tamei. The Mishnah supplies the reason immediately, "mipenei shehatefer mechabero", because the seam binds the two into one. In a saddle the padding is stitched directly into the leather, so we are not dealing with a receptacle plus an independent object lying inside it, but with a single article. Having become part of the leather, the padding bears the leather's tumah, and anyone coming into contact with it is tamei. Naturally, one who touches the leather of the saddle itself is likewise rendered tamei.
The principle that emerges is a clear one. Contents that are merely placed inside a vessel remain their own object and stay tahor even when the vessel around them is an av hatumah. Contents that are stitched into the vessel are no longer contents at all; they are part of the vessel, and they share its tumah completely.