Midrash su Levitico 15:6

1) (Vayikra 15:6) ("And one who sits upon the object that the zav sat upon shall wash his clothes and bathe in water, and he shall be unclean until the evening.") "and one who sits upon the object that the zav sat upon … shall be unclean.": This tells me only of his sitting upon it and touching it. Whence do I derive the same for ten layers (interposing between him and the couch of the zav beneath them, he sitting on top), even if he is sitting atop a stone? It is, therefore, written "And one who sits upon the object that the zav sat upon … shall become tamei." The (same kind of) place that the zav sits upon and confers tumah (to the couch below), the clean man sits upon and becomes tamei (by the couch of the zav below).

2) This tells me only of his sitting there while the zav is (sitting) there. Whence do I derive an equivalence between "empty" (the zav's not sitting there) and "full" (the zav's sitting there)? From "the object" (i.e., the object is the criterion, and not "fullness" or "emptiness"), making "empty" equivalent to "full."

3) This tells me only of the mishkav ([the couch] of the zav, that one who sits upon it becomes tamei to confer tumah upon garments.) Whence do we derive the same for the merkav ([the saddle] of the zav)? It follows by induction, viz.: Just as we find that Scripture did not distinguish between the carrier of and the "carried by" the mishkav (of the zav, so it does not distinguish between the carrier of and the "carried by" the merkav (of the zav). (No, this may be refuted, viz.:) Why did Scripture not distinguish between the carrier of and the "carried by" the mishkav? Because it did not distinguish between touching it and carrying it, (both conferring tumah to impart tumah to garments). Should we then not distinguish between the carrier of and the "carried by" the merkav, where Scripture did distinguish touching from carrying, (the touching not imparting tumah to garments, as opposed to the carrier, who does)! It is, therefore, written "the object," to include the merkav.