ויקרא, פרק ט״ו, פסוק ו׳

Ritual impurity related to certain bodily discharges possesses a unique, penetrating power. It does not rely on direct physical contact, but rather on the transfer of weight and pressure. When a pure person sits or rests on an object previously used by someone with this impurity, the downward force alone passes the impure status onward.

The primary approach among commentators is that this transfer happens even through significant barriers. A person becomes impure even if ten layers of clothing or stacked objects separate them from the impure seat. This principle extends to complex situations, such as a heavy stone resting on pegs directly above the impure seat. Even though the stone itself is immune to impurity, the moment a person sits on it and their weight is supported by the seat below, the impurity transfers to them [שפתי חכמים, מזרחי]. This mirrors the way the impurity is initially created. Just as the original individual transfers impurity downward through layers and stones purely by their body weight, the pure person sitting above absorbs it in the exact same manner [גור אריה, מלבי״ם, אדרת אליהו].

This transfer occurs even if the original person is no longer present, as the object itself has fully absorbed the impure status [תורה תמימה, מזרחי, מלבי״ם]. The laws specifically address the normal, everyday manner in which a person sits [אילת השחר]. The focus is placed on sitting rather than lying down to highlight a crucial point. People typically lie in bed without clothing, meaning any transfer of impurity would happen through direct skin contact. Sitting, however, is usually done fully clothed. This clearly proves that the impurity passes solely through the force of weight and leaning, rather than physical touch [העמק דבר].

Despite the focus on sitting, commentators agree that lying down carries the exact same severity and operates under identical rules. Furthermore, this concept extends to riding gear, such as a saddle, which transfers impurity to a rider in a similar fashion [מלבי״ם, תורה תמימה]. Once a person is affected, the resulting impurity encompasses their entire body. Every part, including their hair and nails, is affected and requires complete immersion in water to achieve purity once again [אדרת אליהו].