Zavim, Chapter 5, Mishnah 5. This chapter continues to work through the ways in which a zav transmits tumah, and in particular through his mishkav and moshav, the surfaces he lies or sits upon. Our Mishnah sharpens one specific question: when a person is only partly on such a surface, does the tumah pass or not?
Two conditions were established already. First, only an item fit to be lain or sat upon can serve as a zav's mishkav or moshav. Second, the item acquires that status only when the zav loads the larger share of his weight onto it. The identical measure governs the next stage: a pure person who later stretches out or sits down on that couch or stool picks up its tumah only if most of what he weighs is being supported by it. Every ruling in this Mishnah is an application of that one yardstick.
The first clause reads "hatamei al miktzas hamishkav", the tamei one on a fraction of the mishkav, "vehatahor al miktzas hamishkav", and the pure one likewise on a fraction of it, and the ruling is "tamei". The zav occupies no more than a sliver of the object, and the pure man too occupies no more than a sliver of it, yet the pure man ends up defiled. The reason lies in what that sliver is carrying. Though only a corner of the couch is in use, that corner is bearing a man in his entirety. The zav's whole weight pressed down there, which is enough to render the object a mishkav, and afterward the pure man's whole weight pressed down there as well, which is enough for him to pick up the defilement.
The opposite arrangement follows: "miktzas tamei al hamishkav", part of the tamei one upon the mishkav, "umiktzas tahor al hamishkav", and part of the pure one upon it, with the verdict "tahor". Here the zav rests only a portion of himself on the couch while most of his weight is carried somewhere else, and the pure man too props only a portion of himself upon it. Nothing is transmitted. The object was never made to support the greater part of the zav, so it never acquired the status of a mishkav in the first place, and whoever subsequently rests against it stays clean.
The lesson of the two clauses together is one clean rule: it is not the percentage of the object that you cover that determines anything, but the percentage of you that the object is holding up. A small piece of wood bearing a whole man is a mishkav; a large bench bearing a fraction of a man is not.
Read side by side, the two halves yield a single crisp measure. What counts is never how much of the object you happen to occupy, but how much of yourself the object is holding up. A narrow plank supporting an entire man qualifies as a mishkav; a broad couch supporting some fraction of a man does not.
A fresh scenario follows: "vechein kikar shel terumah", a loaf of terumah, which is "nasun al gabei mishkav", laid on the mishkav of a zav, "vehaniyar beineihem", with a sheet of paper positioned between the two, and the ruling is "bein milmalah bein milmatah, tahor". Flimsy as it is, that sheet decides the case. It does not matter whether the loaf rests on top with the paper underneath it or the order is inverted; the barrier prevents the loaf from picking up any defilement, and the terumah keeps its purity.
The Mishnah closes with a parallel case and a dispute: "Vechein be'even hamenuga'as, tahor, veRabbi Shimon metamei." Take an even hamenuga'as, a stone from a house afflicted with tzara'as, resting together with a loaf of terumah, whether the stone is above the loaf or below it, with paper separating the two. Here as well, says the Mishnah, the interposing paper protects, and the terumah stays in its purity.
Rabbi Shimon disagrees and declares the loaf tamei. In his view the stone of tzara'as carries a heavier form of tumah, comparable to that of a corpse, and it therefore transmits tumah in the manner of an ohel, overshadowing what lies with it, which a thin sheet of paper cannot stop. That is a stronger power of transmission than what we have been dealing with until now in the case of a zav and his mishkav. The halachah, however, does not follow Rabbi Shimon: the even hamenuga'as does not convey tumah with the force of a corpse, and in the case of our Mishnah the loaf of terumah remains tahor.