Zavim, Chapter 3, Mishnah 3. This chapter examines what happens when a zav and a tahor person share a seat, a perch or a vessel. The concern throughout is that the zav's weight or movement will shift whatever they are both resting on, so that the zav is effectively leaning on the tahor person or moving him. That is heseyt, and where the tahor person is genuinely being sat upon or leaned upon, there is even tumas midras, which renders his very clothing tamei. The earlier mishnayos dealt with unsteady, rickety supports. Our mishnah now turns the picture around and lists case after case where the support is solid, and the conclusion in every one of them is that the tahor person walks away tahor.
The mishnah begins: "Hazav vehatahor sheyashvu bisfinah gedolah", a zav and a tahor person who sat together in a large boat. And what qualifies as a large boat? "Rabbi Yehudah omer: kol she'einah yecholah lehameet ba'adam", Rabbi Yehudah defines it as any boat that a person's weight cannot cause to tip or dip. It is a substantial vessel, and a single passenger shifting his weight simply does not move it.
Next comes a catalogue of surfaces that the two of them might share. "Yashvu al haneser", they sat down on a board; "al hasafsal", on a bench; "al hagoshesh shel mitah", on the frame that runs along the side of a bed; "ve'al hashelonos", or upon beams. To the whole list the mishnah attaches one qualification: "bizman she'einan magirin", provided that they are anchored and do not sway under a person. The comparison with what came earlier is deliberate, since those very same objects appeared in the previous mishnah precisely because they were wobbly there. Here they are tightly secured, and so, for the roomy boat as much as for every fastened object, the verdict is one word: tahor. The zav has certainly not lost his status as a zav, but the man sitting beside him keeps his taharah intact.
The list then climbs higher. "Alu be'ilan shekocho yafeh", they went up into a tree with a sturdy trunk, "uvesochah shekochah yafeh", or out onto a bough whose grip is sturdy. Or the ascent was "besulam tzori", a ladder of the Tyrian sort, a massive and weighty ladder that stands immovably where it is set. Alternatively "o mitzri", the Egyptian variety, which is the slighter and smaller model, and for that one a qualification is required: "bizman shehu kavua bemasmer", that it be pinned down by a nail. Or the two of them were "al hakevesh, ve'al hakorah, ve'al hadeles", standing on a ramp, on a beam or on a door, "bizman shehen asuyin batit", in a case where clay has been used to hold these fast in position. About every one of these situations the mishnah teaches "afilu mitzad echad", that the ruling holds even where both of them went up at one and the same side, so that a tilt would pull in a single direction, and still: "tehorin", the undefiled man and the garments he is wearing stay clear of tumah.
Why? Because none of the mechanisms of tumah are present here. There is no sitting upon the tahor person, so there is no tumas midras. There is no leaning, because the support carries the zav's full weight on its own. And there is no heseyt, because nothing actually shifts or moves the tahor person. The boat is too large to be tipped, the plank and the beams are bolted down, the tree and the ladder and the door are anchored. With no contact, no seating and no leaning, we have no reason to suspect that the tahor person was moved or pressed upon by the zav at all, and he therefore remains fully in his status of purity.
The mishnah closes with two cases of one striking the other. "Hatahor makeh es hatamei, tahor", if the tahor person struck the zav, he stays tahor. Regarding tumas midras this is obvious, since there is no sitting involved at all. One might have thought that heseyt would render him tamei, since when a person delivers a blow he presses and leans into it, and there is real contact. Still, the mishnah rules that he is tahor, and the Rambam in fact holds that he remains tahor even with respect to heseyt.
The reverse case ends differently. "Hatamei makeh es hatahor", when it is the zav who delivers the blow, the mishnah rules "tamei", and the tumah involved is midras, so that the clothing on the man's back becomes tamei along with him. The mishnah spells out the logic: "she'im yimashech hatahor", were the one who was struck to draw himself back and slip out of the way, "harei hatamei nofel", the zav would topple over onto him. That collapse proves that the zav's weight had been resting on the other man all through the blow, and support of that kind is leaning, which brings with it the tumah of midras.
The lesson of the mishnah as a whole is a single principle applied again and again: where the structure beneath a zav is strong enough to hold him entirely on its own, the tahor person beside him has neither been sat upon, leaned upon nor moved, and he stays tahor.