Keilim, Chapter 1, Mishnah 1. This opening chapter functions as a kind of gateway to the whole Seder Taharot. Before the Mishnah turns to the laws of utensils themselves, it lays out the sources of impurity and the ways impurity travels. The chapter is arranged as a climb: it begins with the avot hatumah whose reach is the most limited, and rises step by step to sources of ever greater severity.
An av hatumah is a primary source of impurity, something that can render both people and kelim (utensils) impure. But not every source works the same way. Impurity can pass from one thing to another along several different routes. The most basic is maga, direct physical contact. Others operate through masa, carrying: a person who bears the weight of the impure item becomes impure even without touching it. And others operate through ohel, a shared covering, where merely being under one roof with the source is enough. Our mishnah, together with the mishnayot that follow, sorts the sources of impurity according to which of these routes each one uses.
The Mishnah's List
The Mishnah opens: "Avot hatumot: hasheretz, v'shichvat zera, ut'meh met, v'hametzora bimei sofro, umei chatat she'ein bahem k'dei hazayah." These are the primary sources of impurity that operate through touch alone. Let us take them one at a time.
Hasheretz. The Torah names eight small creeping creatures whose dead bodies are a source of impurity. The carcass of any one of them carries the status of an av hatumah, and whoever comes into physical contact with it, whether a human being or a utensil, is thereby rendered impure.
V'shichvat zera. A seminal discharge from a Jewish male who is at least nine years old.
Ut'meh met. Note the precise wording: not the corpse itself, but a person who has become impure by having had contact with a corpse. That person now carries a status that he can pass along further.
V'hametzora bimei sofro. A person who was afflicted with tzara'at, the skin affliction described in the Torah, during the seven days that he counts toward his purification. Even after the affliction has healed, he must pass through a process of purification that includes bringing the korbanot the Torah prescribes. Throughout those seven counted days he still holds a level of impurity that allows him to render people and utensils impure.
Umei chatat she'ein bahem k'dei hazayah. Mei chatat means water into which the burnt remains of the Parah Adumah have been mixed, the liquid that serves to cleanse a person of corpse impurity. The sprinkling carries a minimum measure: there must be enough liquid that a stalk of hyssop, which is the instrument used for sprinkling, can be dipped in and then shaken over the impure person. Should the combination of ash and water amount to less than that, it has forfeited the very ability to purify for which it was prepared. And here the Mishnah tells us something surprising: water in that deficient state transmits impurity to whoever touches it.
The Rulings
"Harei eilu metam'im adam v'chelim b'maga." All of the items listed convey impurity to a person and to utensils through contact. If a person or a keli actually touches any of them, that person or that utensil becomes impure.
"Uchli cheres ba'avir." With an earthenware vessel there is an additional route. A pottery utensil becomes impure even through its airspace. Picture a jug or a pitcher: one of these sources of impurity enters the hollow interior of the vessel without making contact with any of its walls. It is inside the vessel, yet touching nothing. The vessel is nevertheless rendered impure.
"V'einam metam'im b'masa." But these sources do not convey impurity through carrying. If a person bears their weight while something separates him from them, so that he supports them without touching them, he does not become impure. For the sources in this mishnah, masa alone renders neither a person nor a keli impure. Contact is required.
This is the first rung of the ladder that Masechet Keilim begins to build here: primary sources of impurity whose power extends to touch, and, in the case of earthenware, to the space enclosed within the vessel, but no further.