Ohalos, Chapter 9, Mishnah 15. Here the mishnah leaves coffins of wood behind and moves to solid stone. Imagine a rock formation protruding from the side of a mountain. Someone carves a hollow into that rock, places a meis within it, and then brings a stone slab and sets it across the opening to serve as a covering. The mishnah's question is easy to pose and subtle to resolve: does a person who touches the exterior of that rock become tamei, or does he remain tahor? We will work through the mishnah's language first, and afterwards stand back to lay out the underlying sugya.
The rock that is wide below and narrow above
"Oron sherachovo milmatan vetzoro milmaylan": an aron, which in our case means this hollowed out rock, whose cavity is broad down near the floor and narrows as it approaches the mouth. It makes no difference whether the stone naturally came out that way or whether he deliberately chiseled it so; either way the hollow is shaped like the letter A, spreading out toward the base and drawing in toward the top. "Vehameis besoycho," and the meis is resting inside it.
"Hanoygeia bo milmatan tahor." Someone who touches the outside of this rock down at the bottom remains tahor. Why? Because this stone was not made by anyone. It has been standing there since maaseh bereishis, and bedrock does not carry the din of a kever. Only the man-made portion counts, and the only man-made portion here is the cover that he laid on top. Therefore there are exactly two ways to contract tumah: by touching that cover, or by being underneath it. Since the cavity slopes outward as it descends, a person touching the bottom of the outer wall is standing outside the line of the cover altogether. He is neither touching it nor beneath it, and he stays tahor.
"U'milmaylan tamei." Should he touch the exterior at its uppermost point, he becomes tamei, and obviously he becomes tamei if he lays a hand on the covering itself. Up there he is either in physical contact with the slab or positioned squarely underneath its reach, and it is that slab, fashioned by human hands, which lends the whole setup the status of a kever. Notice, though, how exact the ruling is: he has to arrive at the very highest point of the outer surface before he finds himself in the space beneath the cover.
Wide above and narrow below, and straight walls
The mishnah now inverts the shape. "Rachovo milmaylan vetzoro milmatan": suppose the hollow is broad at its mouth and pinched at its floor, formed like the letter V. Picture an ice cream cone. Grasp the narrow tip and the scoop resting up above is still directly over your fingers. The same logic applies to our rock: every spot on the outer surface lies somewhere within the reach of the man-made covering above it. "Hanoygeia bo mikol mokom tamei": one who touches it at any point, low or high, becomes tamei.
And what if the walls neither slope in nor slope out, but rise straight up, more like the letter U? "Hoysa shova, hanoygeia bo mikol mokom tamei." Since the width is uniform, every point along the wall lies under the cover, and touching it anywhere renders a person tamei. It emerges that there is only one configuration in which a person escapes tumah: the A shape, and only when he touches down at the bottom.
Rebbi Eliezer and Rebbi Yehoshua
"R' Yehoshua omer": Rebbi Yehoshua rejects this. Should a person really become tamei for laying his hand on the mountainside a ten minute climb down from the burial cavity? "Mitefach u'l'matan tahor." Take your measurement from the height at which the meis lies. If the spot he touched on the exterior is a tefach beneath that height, or further down still, he stays tahor. "Mitefach u'l'maylan tamei." If the spot falls inside that one tefach under the meis, or at the meis's height or above it, he is tamei. Picture a laser beam shot horizontally inward from wherever his hand rests: if the beam passes within a tefach of the meis or higher, he is tamei; if it passes lower down, the stone in his hand has no connection to the kever and is nothing but rock.
"R' Yehoshua omer": Rebbi Yehoshua does not accept this. Is a person really tamei for touching the mountainside a ten minute climb below the burial cavity? "Mitefach u'l'matan tahor." Measure from the level of the meis. If he touches the outer surface at a spot a tefach below the meis or lower still, he is tahor. "Mitefach u'l'maylan tamei." If the spot he touches lies within a tefach beneath the level of the meis, or anywhere higher than that, he is tamei. In other words, run a laser line horizontally inward from the point of contact: within a tefach of the meis or above it, he is tamei; below that measure, the stone he is touching has nothing further to do with the kever, and it is simply rock.
Made like a kamtra
"Hu asuyah k'min kamtra." A kamtra is a chest, and what characterizes such a chest is the way its lid is built: it is wide enough to cap not merely the opening but the thickness of the sides as well. Everything until now assumed a plain slab that spans the hollow while leaving the thick rim of stone uncovered on both flanks, so that a hand sliding upward along the exterior never arrives at the covering. With a kamtra, the covering reaches out level with the rim, so that a hand climbing the outer face ends up on the edge of the lid.
In such a case the mishnah rules that the kukh, meaning this grave hewn in the stone, is metamei everywhere: "hanoygeia bo mikol mokom tamei." The Bartenura explains simply that wherever a person touches, he is beneath the cover. But what about the A shape, wide below and narrow above? There, a line drawn upward from the bottom of the outer wall would still miss the cover. The Tosfos Yom Tov answers that once the cover conceals the rim entirely, so that looking down from above you would not see the top surface of the stone at all, the entire stone below becomes part of the kever. The lid and the rock it caps form one unit, and every point on it is reckoned as being under the cover, even where a straight line upward would not reach the slab.
The reverse follows as well. Where the cover does not reach over the walls, we go back to measuring, and if a person touches the bottom of an outward sloping wall so that a line rising from his hand comes nowhere near the slab, he is indeed tahor.
Made like a gloyskis
"K'min gloyskis." A gloyskis is a vessel whose covering is not rested on top but inserted into it. Picture a cork in a bottle: it caps none of the rim, it is driven down into the mouth, with its sides sitting within the opening. If a person carved this burial cavity in the rock along those lines, with a plug lodged in the opening that does not even overlap the inner edge of the rim, then "hanoygeia bo mikol mokom tahor." Touch the stone anywhere at all, including the very peak of the exterior, and you stay tahor, because the covering takes up no space whatsoever above the walls; it is seated down inside the hollow. The single exception is "chutz mim'kom pesichaso," the plug over the opening, which does convey tumah to one who touches it.
The Rambam offers a different pshat in gloyskis. According to him, the meaning is that the chiseling was done from the side, and the meis was slid in horizontally. Here too the ruling is that touching it anywhere leaves a person tahor, since there is no opening on top at all, except for touching the place of the opening itself.
The sugya behind the mishnah: kever sasum
All of this belongs to the subject of kever sasum, a sealed grave with a meis inside it. The classic case of such a grave is this: the meis lies there with a hollow of one tefach by one tefach above him reaching up to the covering, while at his flanks there is no empty space at all, his arms flush against the walls. The halacha in that case is that a sealed grave conveys tumah min haTorah in every direction, on all sides. Whoever passes overhead becomes tamei, including a passenger in an airplane, and this is a live shaylah for kohanim, who need to know where the flight routes run; the poskim take up the question. Whoever stands in a cellar below it becomes tamei too. It is irrelevant that the one above objects that he is not over the meis himself but merely over the grave, or that the one below argues that he is under the grave rather than under the meis. And one who touches the structure from the outside likewise becomes tamei. If, on the other hand, there is no chalal tefach of open space above the meis, we fall back on the rules of tumah retzutzah, where the tumah bursts up and down only along the direct line of the meis.
Our mishnah is a case of a kever hewn into a mountain with a chalal tefach inside, closed by a cover that the person brought and set in place. But our mishnah is different from a standard kever sasum, and the whole difference lies in one point: only the man-made portion carries the din of a kever. The rest is bedrock, standing there from the sheshes yemei bereishis, plain stone. Consequently the only way to become tamei is by touching the cover or standing within its span, and everything in the mishnah flows from that single principle.
Reviewing the mishnah
An oron whose hollow spreads out below and narrows toward its mouth, shaped like an A, with the meis inside: one who touches it low down stays tahor, because a line rising from his hand never meets the cover; one who touches its top, or the cover, becomes tamei. Where the hollow is broad above and pinched below, shaped like a V: tamei at any point of contact, since every spot sits within the reach of the man-made cover. Where the walls go straight up: also tamei at any point of contact. This is Rebbi Eliezer's position, and he would hold it even for someone touching the mountain far down below, provided the vertical line from his hand would pass beneath the cover. Rebbi Yehoshua narrows it: a person can still become tamei at a spot lower than where the meis rests, but only within one tefach; anything under that measure leaves him tahor, and from that tefach on upward he is tamei.
All of this concerns a cover that does not extend over the rim. Where it does, made like a kamtra, he is tamei wherever he touches, and even in the A shape the Tosfos Yom Tov holds him tamei, because a cover that conceals the entire rim draws the whole stone beneath it into the kever. And where the cover is wedged into the opening like a cork, made like a gloyskis, he is tahor wherever he touches, since he is guaranteed not to be beneath the cover, except at the place of the opening itself; the Rambam understands gloyskis as a cavity cut from the side, with no opening above at all, where again he is tahor everywhere but at the opening.