Zavim, Chapter 3, Mishnah 1. This chapter turns to a very practical question: what happens when a zav and a tahor person end up together in one confined place, or on one shaky object, in circumstances where neither of them can be sure whether they brushed against each other or leaned on one another. Our mishnah divides neatly into two parts, and both of them revolve around that same concern.
Part One: A Boat, a Raft, an Animal
Our mishnah opens with a case of shared travel: "Hazav vehatahor sheyashvu bisfina", a zav and a tahor man seated together in a boat; "o va'asda", or on a raft; "o sherachvu al gabei beheima", or riding together on an animal. The mishnah then grants us full knowledge that no garment of the one ever met a garment of the other: "af al pi", even in a case where "she'ein bigdeihem nog'im", there was no contact at all between their clothing. The ruling stands regardless: "harei eilu temei'im midras", the clothing of the man who was tahor is regarded as having contracted tumas midras, with all the severity of an av hatumah.
The obvious question is why. If we know that the clothing of the two men did not come into contact, on what basis do we declare the tahor person's garments tamei? The answer lies in the setting. A boat rocks, a raft bobs and shifts, an animal sways with every step. Two people sharing such a space are constantly being jostled forward and back and from side to side. And these are not spacious vessels; the mishnah is describing a small boat or a modest raft that barely holds two passengers, and an animal that is hardly a six-seat carriage. Under those conditions the odds are overwhelming that at some point the zav pressed against the other man's clothing, and pressing of that kind is exactly what generates midras. This is essentially how the Rambam understands the ruling.
Others, among them the Rosh and the Rash, offer a second line of reasoning. Suppose the tahor man happened to be the heavier of the two. His weight would have caused him to settle lower, so that the zav's garment came to rest above him and was in effect borne and supported by him. That relationship of weight and support is itself enough to render his garments subject to tumas midras.
So we are left with two ways of accounting for the ruling: either the two men almost certainly made contact as the vessel rocked and shifted, or the difference in their weights produced a situation in which one was supported by the other. Either way, the garments of the man who began the journey tahor emerge from it bearing midras.
Part Two: Unsteady Objects
A second list follows. "Yashvu al haneser", they sat down together on a plank; "al hasafsal", on a bench; "al hagatshish shel mita", on the framework of a bed (some understand the term to mean the small blocks of wood set beneath a bed's legs); "ve'al ha'achlonas", and on a beam. To each of these items the mishnah attaches an indispensable condition: "bizman sheheim machgirin", that is, they had never been fastened down as they should have been, and so they wobbled and rocked beneath the two men.
The list runs on. "O alu be'ilan shekocho ra": or the pair climbed into a tree of feeble build, the kind that sways heavily once someone is up in it. "Kocho ra be'ilan yafeh": or they were resting on a frail limb belonging to an otherwise sturdy tree, a limb that bends and shifts under a person's weight. Next comes a ladder of the Egyptian type, meaning a short and lightweight one, "o sulam mitzri", together with its qualification, "bizman she'eino kavua bemasmer", namely that it had not been fixed to the wall with a nail. Then "al hakevesh", on a ramp; "ve'al hakora", on a rafter; and "deles bizman she'eino asuy betit", on a door that had not yet been set firmly into place with clay. Wherever the two of them found themselves together atop something defective that rocked and gave way under them, the verdict of the Tanna Kamma is "temei'im": tamei.
The reasoning is once again a matter of probability. A man perched on such a flimsy object is pitched back and forth, and it is likely that in the course of all that movement the zav came into contact with him and conveyed tumah to him.
"Rabbi Yehuda metaher": with respect to every item on this list, Rabbi Yehuda parts company with the Tanna Kamma and holds that the man who was tahor keeps his taharah intact.
Two Different Levels of Tumah
It is important to notice that the two halves of the mishnah are not speaking about the same status. The opening cases, the boat and the raft and the animal, produce tumas midras. The second list produces what is called tumas heseis, the tumah that arises from a probability that the zav shifted against the other man.
Why the difference? The first set of cases involves objects that are perfectly solid, but very small. Two people crammed into a tiny space are almost bound to make contact. Add to that the element of time: a person may be in a boat or on a raft for many days on end. An extended stay in a confined place makes contact something close to a certainty, and the resulting tumah is the full tumah of midras. The second set of cases involves rickety, unstable objects on which a person typically spends only a short while. There the likelihood of contact is much lower; we are merely choshesh, we take the possibility into account, and the result is the lighter category of tumas heseis, which carries its own set of rules.
The halachah does not follow Rabbi Yehuda. Even in the cases of the wobbly plank, the weak branch and the unsecured door, where the two men were together only briefly, we are concerned enough about contact to declare the man tamei.