QUESTION: The Gemara cites a Beraisa which states that although the Torah forbids one only from wearing Kil'ayim upon his body, the Rabanan prohibited even lying or sitting on Kil'ayim out of concern that perhaps a thread of Kil'ayim will become wrapped upon the person in such a way that he will benefit from wearing it. The Gemara asks that perhaps the Isur d'Rabanan applies only when the Kil'ayim is directly underneath the person, but if another object separates between him and the Kil'ayim he may lie or sit on it. The Gemara responds with a statement of the Tana'im that even when ten sheets are spread over the Kil'ayim, one sheet on top of the other, one may not lie on the uppermost sheet.
Why does the Beraisa say that the reason for the Isur d'Rabanan against lying on Kil'ayim is because of the concern that a thread of Kil'ayim will wrap up onto the person's body? If one is forbidden to lie even on top of sheets which cover the Kil'ayim, the prohibition obviously is not due to the concern that a thread might wrap up onto one's body, because many sheets separate his body from the Kil'ayim! Rather, the Isur is merely a Gezeirah to prevent one from taking the Kil'ayim from underneath him and wearing it. Why, then, does the Beraisa say that the reason is because a thread of Kil'ayim might become wrapped around part of his body?
(a) The RITVA here says that the Gezeirah which prohibits lying on top of ten sheets which are on top of Kil'ayim is actually a "Gezeirah l'Gezeirah," a rabbinical decree made to safeguard another decree. The original Gezeirah of the Rabanan prohibited lying directly on a garment of Kil'ayim lest a thread of Kil'ayim wrap up onto his skin. The Gezeirah that prohibits lying on the uppermost of ten sheets which are on top of Kil'ayim was enacted in order to prevent one from lying directly on Kil'ayim, which in turn is prohibited lest a thread wrap onto his skin. (The two Gezeiros are considered to be one Gezeirah -- "Kula Chada Gezeirah" -- since they were enacted at one time.)
(b) The RAMBAM (Hilchos Kil'ayim 10:12) appears to understand that even if a garment of Kil'ayim is underneath ten sheets, there still is a concern that a thread from the Kil'ayim might reach the top of the sheets and wrap around one's skin. Accordingly, only one Gezeirah was enacted. This also appears to be the opinion of TOSFOS (DH Ela b'Kashin, and in Yoma 69a, DH Kashin).
The Rambam and Tosfos are consistent with their own opinion as expressed elsewhere. They maintain that one is prohibited from lying directly on top of a garment of Kil'ayim even if it is very hard. One is permitted to lie on top of it only when another object separates between his skin and the hard garment of Kil'ayim. They consider sitting on Kil'ayim a form of "Ha'ala'ah," placing the garment on one's body, because there is no logical reason to differentiate between a garment on top of one's body and one's body on top of a garment. As long as one's flesh touches the garment, it is considered "Ha'ala'ah." According to this opinion, no Gezeirah is necessary to prohibit one from sitting on Kil'ayim because of the reason that a thread might wrap up onto his body, since the prohibition of Kil'ayim itself prohibits him from sitting on Kil'ayim. Why, then, was such a Gezeirah enacted? It must be that the Gezeirah that a thread might wrap up onto one's body was enacted in order to prohibit him from lying on a garment of Kil'ayim when it is underneath several sheets.
(c) The RAN infers from the words of RASHI (DH Shema Tikarev) that the prohibition against lying on Kil'ayim lest a thread wrap up onto one's body is an Isur d'Oraisa and not merely d'Rabanan. When the Beraisa teaches that "the Chachamim said that it is forbidden [to lie on Kil'ayim]," it means that it is forbidden because it is a doubt in a case of an Isur d'Oraisa. In the case of Kil'ayim underneath ten sheets, there is no Isur d'Oraisa but only a Gezeirah that one not come to sit on the garment of Kil'ayim itself.
(d) The RAN himself, however, suggests that the Gezeirah against lying on top of sheets which are on top of a garment of Kil'ayim is a Gezeirah to prevent "Ha'ala'ah" -- to ensure that one does not pick up the garment and wear it. It is unrelated to the concern that a thread will wrap onto one's body. There is no concern for a thread in this case because the Kil'ayim is separated from his body by other sheets.
The concern that one might pick up and wear the Kil'ayim, however, applies only to a type of garment that is normally worn, but not to sheets. If the Kil'ayim underneath the sheets is itself a sheet, one is prohibited only to sit directly on top of it. There is no concern that he might pick it up and wear it, but rather there is a concern that since he is sitting directly on the Kil'ayim a thread might wrap up onto him. In contrast, one may not sit on an item of clothing of Kil'ayim even when it is underneath ten sheets, because in such a case the Gezeirah that he might wear it applies.
This explanation is supported by the text of the Beraisa which says, "Even ten sheets, one on top of the other, and Kil'ayim beneath them [is prohibited]." This implies that the prohibited item itself is not a sheet but a different type of garment (a wearable one) made of Kil'ayim. (See also Insights to Yoma 69:2.)