Ohalos, Chapter 12, Mishnah 4. Our Mishnah presents a case of an upper story and a lower story, with tumah below and a household above, and asks how much of an opening between the two floors it takes for the tumah to travel upward. Along the way the Mishnah teaches us something precious about how tumah is measured out when it does not spread as tumas ohel.
The Mishnah begins: "sandal shel arisa", the sandal of a crib. A sandal is ordinarily a kind of shoe, and that is exactly the picture here, except that this shoe does not belong to a person. It belongs to a crib. In those days they would fit little caps or shoes onto the bottoms of the legs of a crib. Sometimes it was simply attractive, a bit of decoration, and sometimes it served a practical purpose: it kept the ends of the wooden legs from sitting directly on the floor and rotting away.
Now picture that crib standing in a room on the upper floor. Over time the metal or wooden shoe on the leg pressed and ground against the floorboards beneath it, until it wore its way clear through. That is the Mishnah's language: "shepochasoi b'toich habayis", that it broke through into the house below. The hole is a genuine hole, right through the floor. Someone standing in the lower room can look up and actually see the crib's sandal poking through from above.
Tumah below, a crib above
And in that lower room, unfortunately, lies a meis. So a person standing next to the corpse downstairs raises his eyes and sees the sandal of the crib protruding through the ceiling. The question is what happens to the upper story.
The Mishnah rules: "im yesh bo poseach tefach, hakol tamei". Should the gap measure a square tefach, everything is tamei. A breach of such dimensions joins the two levels into one single ohel, so the tumah below climbs upward and the whole story above becomes tamei.
"V'im lav": should the breach fall short of a square tefach, the tumah stays where it is and never reaches the story above. Yet that is no reason to imagine that the utensils upstairs escape entirely unaffected. The Mishnah adds: "moineh bo k'derech shemoineh b'meis", we reckon here exactly as we reckon with a corpse.
Counting instead of spreading
What is this reckoning? It signals that tumas ohel is no longer in play, where an entire enclosed area and all its contents are caught up at once at one identical grade. In its place we invoke the standard laws of tumah conveyed by touch: whatever ordinarily results when a tamei item touches something else is what results here. And touch operates on a graded ladder: the original generator of tumah stands highest in severity, and every further handoff lowers the rank by one rung.
In our situation, then, the sandal juts into the room below, but the gap does not reach a tefach. Nothing travels upstairs by tumas ohel, and we do not pronounce every vessel of the upper story tamei in one stroke. We proceed instead link by link. The sandal is a keli capable of receiving tumah, and it was located within the ohel hameis, so it contracts tumah. It then conveys tumah to the crib, not through tumas ohel but through plain touch. The crib in turn conveys tumah to the infant asleep upon it.
And this is precisely the point: the three of them do not share an identical rank. Were there a poseach tefach, tumas ohel would have swept through the room above and placed everything in it at one uniform grade. As it is, tallying rung by rung, the sandal and the crib acquire tumas shivah, a seven day tumah, whereas the child carries a tumah of that single day alone. A seven day tumah calls for the sprinkling of the waters of the Parah Adumah (its ashes mingled with spring water) on day three and again on day seven, with tevilah at the end. Why the child gets off more lightly deserves explanation.
Cherev harei hu k'chalal
How did the sandal become tamei? By sitting inside the ohel hameis. And what rank does that give it? The rank of avi avos hatumah. One could challenge this: surely the corpse itself occupies that top rank, so the sandal ought to slip down a notch. The answer lies in the rule of "cherev harei hu k'chalal", a blade is treated like the corpse it touched. A utensil that has touched a meis keeps the identical grade borne by the body itself. Since a meis stands as avi avos hatumah, the sandal that lay beneath the same ohel as the corpse holds that rank as well.
This extraordinary rank, however, does not travel further down the chain. A utensil that touched the corpse itself holds the rank of avi avos hatumah, but a second utensil that touches that first one slides down a step and becomes an av hatumah. Only direct contact with the body confers the highest rank. Hence: the sandal, which stood within the ohel sheltering the corpse, is avi avos hatumah, while the crib, which the sandal touched, is av hatumah. Both grades, avi avos and av alike, oblige a seven day procedure of sprinkling and immersion.
The child, by contrast, received his tumah from an av hatumah, and contact with an av hatumah does not impose a tumah of seven days. His tumah lasts through that day alone. This is the force of "moineh bo k'derech shemoineh b'meis": when the gap is under a tefach, no single blanket tumah is laid over the upper story; rather we trace the chain of contact one stage at a time, and every stage takes on exactly the grade that belongs to it.