Kelim, Chapter Nineteen, Mishnah Four
1) If a zav was carried on a bed and on its mattress, the latter causes an uncleanness of two grades and an unfitness of one grade, the words of Rabbi Meir.
2) Rabbi Yose says: if a zav was carried on a bed and on its mattress the part that is shorter than ten handbreadths causes an uncleanness of two grades and an unfitness of one grade, but that which is over the ten handbreadths causes only an uncleanness of one grade and an unfitness of one grade.
3) If he was carried on the mattress, [on the overhanging part] that was shorter than ten handbreadths, it becomes unclean, but if on the part that was longer than ten handbreadths it remains clean.
Section one: The zav (person with abnormal genital discharge) causes the mattress to become a primary source of impurity (the bed also is a primary source of impurity, but this mishnah deals only with this mattress). The mattress causes anything that touches it to have first degree impurity. Whatever has first degree impurity imparts second degree impurity to whatever it touches. That which has second degree impurity renders unfit any terumah that it might touch (giving it third degree impurity). But the terumah does not disqualify anything by giving it fourth degree impurity.
Section two: Rabbi Yose distinguishes between the part of the mattress that extends from the bed less than ten handbreadths and the part that extends more than ten handbreadths. This accords with what he stated in yesterday’s mishnah. The part that extends less than ten handbreadths has the same rule that Rabbi Meir stated above. The part that is further away from the bed is treated not like the bed but as if it were a separate piece that had contact with the remainder of the mattress. Therefore, this part has first degree impurity which it got from the main part of the mattress and it defiles that which it touches, giving it second degree impurity. That which has second degree impurity renders unfit any terumah that it might touch (giving it third degree impurity).
Section three: In this scenario he was carried on the part of the mattress that extends over the bed, but not on the bed itself. If he was carried on the part of the mattress that is less than ten handbreadths from the bed, the mattress is unclean. But if further from the bed, then the mattress is clean because it is not susceptible to “midras” impurity (imparted by sitting, lying or leaning on something) because it is not considered part of the bed. And since the zav did not directly touch the mattress (he touched the mattress covering), the mattress is completely clean.