Zavim, Chapter Four, Mishnah Seven
1) If he sat on a bed and there were four cloaks under the four legs of the bed, all become unclean, since the bed cannot stand on three legs;
a) But Rabbi Shimon declares them clean.
2) If he rode on a beast and there were four cloaks under the legs of the beast, they are clean, since the beast can stand upon three legs.
3) If there was one cloak under its two forelegs or its two hindlegs, or under a foreleg and a hindleg, it becomes unclean.
4) Rabbi Yose says: a horse conveys uncleanness through its hindlegs and a donkey through its forelegs, since a horse leans upon its hindlegs and a donkey upon its forelegs.
5) If he sat on a beam of an olive-press, the vessels in the olive-press receptacle are unclean.
6) [If he sat] on a clothing press, the garments beneath it are clean.
a) Rabbi Nehemiah declares them unclean.
Section one: Since the bed can’t stand without all four legs, we have to look at each leg of the bed as if it carries the full weight of the zav. Therefore, all four cloaks are impure because he sat on all four of them.
Rabbi Shimon says that all four of the cloaks are clean. This is because none of them bears the zav’s full weight. Since his weight is evenly distributed among the four legs, they all remain pure.
Section two: In contrast to a table, an animal can stand on three legs, so any one leg is not necessary for bearing the weight of the zav.
Section three: But a beast cannot stand on two legs. Therefore, if the cloak is underneath any of the two legs, then it is considered to bear the full weight of the zav and it is unclean.
Section four: According to Rabbi Yose a horse bears its weight on its hindlegs and a donkey’s weight is borne on its forelegs. Therefore, if the cloak is under the horse’s hindlegs or the donkey’s forelegs it is impure. But if not, it remains pure.
Section five: Since the zav presses down on the olive press, the olives are impure.
Section six: Unlike an olive press, a clothing press is already closed tight. When the zav sits on the clothing press, he doesn’t really press down on the garments. Therefore, they remain clean.
Rabbi Nehemiah holds that the zav does press down on the clothing press and the clothing inside and therefore they are defiled.