Commento su Levitico 15:6

והישב על הכלי AND HE THAT SITTETH ON ANY THING [WHEREON HE SAT THAT HATH AN ISSUE … SHALL BE UNCLEAN] — even though he (the person) is not actually in contact with it: even if there be ten things one upon the other, the lowest being that upon which a זב has once sat, they all communicate uncleanness according to the law applicable to a seat upon which an unclean person has sat, and similarly is the case with an object upon which he has lain (cf. Sifra, Metzora Parashat Zavim, Chapter 3 1).

Even ten vessels one on another. As it is written, “Whoever sits on the vessel that was sat upon by a zov.” This makes a comparison between the sitting of the pure person and the sitting of the zov: Just as the sitting of the zov causes impurity to the vessels even though he does not sit on the vessel, such as the case of the “placed” stone (see later), so too is the sitting of the pure person who acquires impurity from the vessel which contracted impurity from the zov, even if he does not touch it. A “placed” stone is a stone placed on pegs with vessels underneath it, as it is written (Daniel 6:18): “And a stone was brought and placed on the mouth of the pit.” If a menstruant woman or a zov sit on it, even though their weight is not felt on the vessels, the vessels are impure, as it is written (v. 10): “Anything which is beneath him.” Rashi (Shabbos 82b).

And so [too] in regard to a bed. [Rashi knows this] because we derive through a מה מצינו (comparison) from sitting. Meaning: If a pure person lies down on a “placed” stone (see above) and a zov’s bed was underneath the stone, the pure person becomes impure even though the stone separates between them, and the stone does not acquire impurity; nonetheless, he is impure. Although it is written only regarding sitting, we derive lying down from it as well, since lying down and sitting are equal in their severity of touch, to cause impurity to clothing, [thus,] they are one matter.