Keilim, Chapter 2, Mishnah 1. The Torah teaches that keilim can contract tum'ah, whether from a person who is tamei or from a tamei object. A keli only becomes tamei from a high-level source: an av hatum'ah or an avi avot hatum'ah. But the details of how it becomes tamei, and what tum'ah it carries, depend entirely on what the keli is made of.
This is the gateway to the next major section of the masechet. The coming eight chapters are largely devoted to k'lei cheres, earthenware utensils, which differ in several important respects from all other keilim. In order for those differences to stand out, our mishnah first lays down the general rules that govern the other materials, and only then contrasts them with earthenware.
The four materials
The mishnah's list begins with four substances: "K'lei eitz" (utensils of wood), "uch'lei or" (of leather), "uch'lei etzem" (of bone), and "uch'lei z'chuchit" (of glass). A single body of halachah governs all four together.
Two clarifications are in order. A utensil of bone means one carved out of bone, and what is meant is the bone of land animals; the bones of fowl and of fish are not included. A utensil of leather is one produced from the hide of an animal that lives on land, and here too the hides of birds and of fish fall outside the category.
Glass occupies a category by itself. On the level of Torah law glass vessels are not susceptible to tum'ah at all, since glass is not one of the substances the Torah enumerates. Their tum'ah was instituted by Chazal, and the logic behind the decree is worth noting: since glass is manufactured out of sand, it bears a likeness to cheres, and vessels of cheres do carry tum'ah min haTorah, as do those of wood, leather and bone.
Flat versus receptacle
For all four substances the ruling reads: "P'shuteihen tehorim", when such utensils are flat they remain tahor, "um'kab'leihen t'mei'im", and when they are capable of containing, they are susceptible. Susceptibility, then, depends on the presence of a beit kibul, a hollow that genuinely holds whatever is put in it. There must be sides and an edge, so that the contents stay where they are placed instead of sliding off.
A flat wooden tray, or a stiff flat sheet of leather, has no receptacle at all, and therefore keilim of this shape, made from these materials, simply cannot contract tum'ah. That is the Torah's law.
On a rabbinic level there are cases where even flat keilim of wood, leather and bone do become tamei. Glassware, however, does not become tamei when flat even by rabbinic law, and the same is true of flat earthenware keilim.
Broken, and rebuilt
The mishnah goes on: "Nishb'ru taharu". Should a keli of this kind that had become tamei then shatter, its tum'ah departs. Precisely how far the damage must go is a question the coming mishnayot will address; but a utensil reduced to small pieces has certainly lost whatever tum'ah it carried.
And it adds: "Chazar v'asah meihen keilim", should someone take those fragments and fashion a utensil out of them, then "m'kab'lin tum'ah mikan ul'haba", that utensil is liable to tum'ah from now on. The tum'ah it once held does not come back. Halachically it is a fresh keli beginning its life clean: tahor at this moment, and capable of becoming tamei in the future.
Earthenware and neter
The mishnah now turns to the substance that will dominate the chapters ahead: "K'lei cheres", utensils of earthenware, together with "v'chlei neter", utensils of neter, of which it says "tum'atan shava", their halachot of tum'ah are one and the same. Neter is a certain variety of soil or clay, and its exact identity is a matter of machloket; some understand it to be chalk. Whatever its identity, it too can take on tum'ah, and it does so exactly along the lines laid down for earthenware.
Four halachot are then set out, shared equally by the two.
1. Airspace
"Mitam'in um'tam'in ba'avir": these utensils both receive tum'ah and transmit it through the air enclosed within them, with no contact whatsoever. Imagine a sheretz suspended inside the hollow of such a keli, present in its space yet never resting against its floor or its walls: the keli is tamei all the same. In the reverse direction, food held aloft inside the inner space of a tamei keli of cheres or neter, in contact with nothing, becomes tamei. Tum'ah enters and departs through air alone.
No other substance shares this feature. Every other keli acquires tum'ah only by actual contact, whether it is touched on its inner or its outer surface; the ability to absorb tum'ah out of the surrounding air belongs to cheres and neter alone.
2. The hollow underside
"Um'tam'in mei'achoreihen": tum'ah can also reach them by way of the recessed cavity on the underside. Picture a vase whose base, the part that stands on the table, curves inward and leaves a shallow pocket. Turn the vase upside down and that pocket serves as a small container of sorts. Should something tamei come into contact with the inner face of that recess, the keli is rendered tamei.
Note the precise language: touching. Here, unlike the inner space of the keli, airspace alone is not enough. Since this inverted underside is not ordinarily put to use, neither as a stand nor as a container, actual contact is required. This tum'ah is a rabbinic tum'ah, and the rabbis limited their decree to contact and did not extend it to airspace.
3. Not from the outside
"V'einan mitam'in migabeihan": the outer surface is not a route by which they acquire tum'ah. If something tamei comes up against the outside wall of a keli of cheres or neter, the keli stays tahor. Such utensils are rendered tamei only from within, either by contact with the inner surface or by tum'ah suspended in the inner space, and this halachah is derived from a pasuk.
Once such a keli has become tamei, however, it certainly transmits tum'ah through its outer surface as well, and through its airspace.
4. Breaking is the only purification
"Ushviratan hi taharatan": breaking them is their only path to purity. An earthenware or neter keli cannot be purified by immersion in a mikveh. When it is broken to the point that it no longer qualifies as a keli, it loses the tum'ah it had. And there is more: even if the pieces are subsequently repaired and reassembled, the item can never again become tamei. Once it has been broken, it has permanently lost the status that would make it a keli. Breaking is the only remedy, and it is a final one: from that moment onward the item is beyond the reach of tum'ah forever.
With these four rules the Mishnah has drawn the sharp line between earthenware and every other material, and set the stage for the chapters that follow.