Zavim, Chapter 4, Mishnah 6. This chapter has been exploring the tumah that a zav conveys by heseit, by shifting or moving an object even without touching it, and our mishnah continues with a case of a balance scale: a zav resting in one pan, and something else in the pan opposite him.
The mishnah begins: "Hazav bekaf moznayim," a zav who is in one pan of a balance, "ve'ochlin umashkin bekaf sheniyah," with foods and beverages in the pan across from him, "tmei'in," they are tamei. Their level is that of a rishon, a first degree of tumah. What brought this about? The zav's heseit. His weight pushes his own pan down and thereby lifts and shifts the opposite pan, and once a zav sets something in motion that is sufficient to defile it. It is worth being precise here: we are not dealing with midras. Midras attaches only to items whose purpose is to be sat or lain upon, and a piece of bread or a cup of wine clearly has no such function.
The second half of the ruling reverses the case. "Uvameis," where a corpse occupies one pan, "hakol tahor," nothing in the facing pan contracts tumah, "chutz min ha'adam," with the sole exception of a person. Place in that second pan whatever you wish: a bed, a chair, bread, oil, any utensil at all. It stays tahor, and that holds true even when the corpse outweighs everything else, so that the meis itself is what does the lifting. The reason is straightforward: a corpse does not defile by setting things in motion. Its tumah requires either real contact or the tumah of ohel, sharing a covering with the corpse.
There is one exception, and that is "chutz min ha'adam," a person. A human being who moves a meis becomes tamei by that very act of carrying and shifting it. Therefore a person sitting in the second pan, who never laid a finger on the corpse, is nevertheless tamei, since through the workings of the scale he has moved it.
At this point the mishnah draws up a two sided ledger: "zeh chomer bazav mibameis," this is a respect in which the zav is the graver of the two, "vechomer bameis mibazav," and there are also respects in which the corpse is graver than the zav. Certain severities belong to the tumah of a zav and have no parallel by a meis, and certain severities belong to the tumah of a meis and have no parallel by a zav.
The stringency of the zav
The mishnah spells out the zav's side: "shehazav oseh mishkav umoshav," the zav produces a mishkav and a moshav out of what is "mitachtav," positioned under him, and such an object goes on "letamei adam uletamei vegadim," to defile a person and to defile clothing. Anything suited to being lain or sat upon that finds itself beneath him acquires midras tumah, and no touching between the zav and that item is required. Someone who later handles or lifts that mishkav becomes tamei, and for as long as he remains joined to it he passes tumah on to vessels and to clothing as well. Imagine the zav stretched out on a tall column of cushions, or on a mound of coats: the uppermost, the lowest and every layer between them are all rendered tamei, each one able to defile a person, and that person then defiles what he is wearing.
The mishnah turns to what lies over him: "ve'al gabav madaf," whatever is above the zav becomes madaf. A thousand items heaped over him are included, and so are items with no connection at all to sitting or lying. Yet this madaf carries a much weaker force. It is merely a rishon, and its reach extends "letamei ochlin umashkin," to defiling foods and drinks alone, while human beings and vessels remain unaffected by it. Compare that with a mishkav or moshav under him, which does defile people and clothes. All of it, says the mishnah, is "mah she'ein hameis metamei," something a corpse does not accomplish.
How the meis differs
With a meis the picture is entirely different. Items positioned over it or under it do not become tamei whatsoever; only that which makes actual contact with the corpse is affected. Say a number of vessels are lined up next to it: the meis, an av hatumah, defiles the one vessel it touches, and that vessel becomes a rishon, while the vessels standing further along remain tahor. Layer upon layer of tumah is transmitted only through mishkav and moshav; a vessel, by contrast, can be defiled solely by an av hatumah.
The stringency of the meis
Here the corpse proves to be the heavier of the two. Its tumah travels through an ohel, so that human beings and utensils under one roof or one covering with it become tamei. And it is "umetamei tumas shivah," it imposes a seven day tumah: whoever touches a corpse or carries it remains tamei for seven full days. This is "mah she'ein hazav metamei," something the zav does not do. His tumah lasts no such stretch of time, and the tumah of ohel has no application to him whatsoever.
So the mishnah leaves us with a balanced picture: the zav reaches objects he never touched, above him and below him, while the meis reaches only what touches it, yet with a severity of ohel and of seven days that the zav never has.