Zavim, Chapter Four, Mishnah Four
A zav who lies on something makes it unclean. Our mishnah deals with cases where a zav lies on multiple itemsdo they all become impure, or can we isolate which one(s) he really laid upon?
1) A zav who lays across five benches, or five money-belts:
a) If lengthwise, he [makes them] unclean;
b) But breadthwise, they are clean.
2) If he slept [on them], and it was doubful that he had turned over onto them, they are unclean.
3) If he was lying on six seats, with two hands on two [seats], two feet on another two, his head on one, with his body on another one, only that one on which his body lay is rendered unclean.
4) If [a zav] stood on two seats: Rabbi Shimon says: if these were distant one from the other, they remain clean.
Section one: If the zav lies across the length of five benches or money-belts it is possible that at any one time most of his weight was on one of the benches or belts. Therefore, they are all impure.
However, if he lies on them across their breadth then his weight is distributed equally among them all. Since he didn’t lie on any one of them, they all remain pure.
Section two: If he slept on the benches or the money-belts then they are impure even if he slept across their breadth because we don’t know if he turned around a lot while sleeping. Since he might have put all of his weight on one, they are all impure.
Section three: Since the majority of his mass was on one of the seats, only that seat is unclean.
Section four: If the two seats are far apart from the other, then the zav would need both of them to stand on. Take one away and he would fall down. Since neither would support the full weight of the zav each remains pure. But if they were close together, at times the zav could shift all of his weight to one and then it would be defiled.