Keilim, Chapter 1, Mishnah 4. Our chapter is building a ladder. Step by step the Mishnah lists the avot hatumah, the primary sources of impurity, arranging them from the lighter to the heavier, and at each rung it tells us exactly what the higher source can do that the one beneath it cannot. This mishnah continues that climb, and it carries us all the way to the top.
A Zava Is More Stringent Than a Zav
"Lemala min hazav, zava": the rung directly above the zav belongs to the zava, a woman whose flow has given her that status. Whatever impurity the zav imparts, she imparts as well, and there is one power that is hers alone: "shehi metame'a et bo'alah," her bo'el, the man who lies with her, is himself rendered impure, and not merely impure. He becomes an av hatumah, a primary source of impurity in his own right.
Nothing equivalent exists on the zav's side of the ledger. If a woman lies with a zav, the act itself grants her no special standing. She does become impure, naturally, since his body has been in contact with hers, but this is nothing more than the ordinary impurity of touch: she is a rishon letumah, a first degree, no different from anyone else who happened to touch him. The intimacy contributes nothing beyond that. The zava, on the other hand, turns her bo'el into an av hatumah, and it is precisely this that earns her the higher place.
A Metzora Is More Stringent Than a Zava
"Lemala min hazava, metzora": above the zava stands the metzora, the person afflicted with tzara'at. The metzora conveys impurity in every manner we have already described, and he adds a new one: "shehu metameh be'vi'ah," he conveys impurity by entering. When a metzora comes into a house, or into any roofed and covered space, a person or a utensil that is inside at that moment becomes impure.
Notice how far this reaches beyond everything listed until now. There is no need for touching, no need for leaning, no need for carrying. The mere fact that the metzora and the person or utensil find themselves together beneath one covering is enough. One condition does apply, and we will return to it shortly: the metzora must actually come to a halt there. Passing through the space is not sufficient. He must stand still for a moment, and then the covering that unites them transmits the impurity.
A Bone the Size of a Barley Grain Is Stricter Still
"Lemala min hametzora, etzem kise'orah": higher than the metzora we find a bone taken from a corpse, provided that it measures a barley grain or more. Its added severity concerns how long the impurity endures: "shehu metameh tum'at shiv'ah," whoever or whatever it renders impure remains so for seven days. Both touching such a bone and carrying it bring about this result.
Compare that to the metzora. A person or utensil that becomes impure through a metzora becomes a rishon letumah, an impurity that lasts a single day. The bone imposes a full seven days, and a person who contracts it must undergo the entire purification process of the parah adumah before he is clean again.
It is worth pausing to appreciate that this ladder is not absolute in every respect. The bone is not stricter than the metzora across the board. The metzora, as we have seen, conveys impurity to whoever shares a covered space with him, while a barley-sized bone has no such power at all: it does not transmit impurity through a shared roof. In that respect the metzora is the more stringent of the two. What the Mishnah is measuring here is a different axis, the length of the impurity, and along that axis the bone stands higher.
The Corpse Is the Most Stringent of All
"Chamur mikulam hameit, shehu metameh be'ohel": more stringent than everything listed is the corpse itself, for it conveys impurity through an ohel, a tent or covering. A corpse renders a person or a utensil impure not only by being carried and not only by being touched, but through the simple fact that both are found beneath the same covering, whether a roof, the branches of a tree, or anything comparable that spreads above them.
And the ohel of a corpse is not the same as the entry of a metzora. With a metzora, only when his entire body is beneath the covering together with the person or the utensil does the impurity take effect. With a corpse, the whole body is not required. Even a part of it under the covering suffices to render impure whoever is there together with it.
There is a further difference. The metzora transmits impurity only when he comes to rest, when he stands stationary beneath the covering together with the other person. Should he merely pass through, nothing is conveyed. A corpse is not bound by this condition at all: even if it is carried through the ohel and never stops for a moment, it conveys impurity to whatever is inside.
This is the force of the Mishnah's closing words, "mah she'ein kulan metam'in": none of the other primary sources of impurity listed before it conveys impurity in so far-reaching and severe a manner. The corpse stands at the summit of the ladder that this chapter has been climbing, and every rung beneath it is measured against what it alone can do.