仁王經疏 3.156

3.156

釋曰：第二、修不淨忍。自有三種：一觀六大不淨、二觀諸根不淨、三觀三界不淨。此觀六大，謂諸有情六界成身，所謂四大及空界也。色界有漏識，皆是有漏，可破壞故，名為不淨。六界別觀，故言分分。

Explanation: Second is cultivating the patience of impurity. This itself has three kinds: first, contemplating the impurity of the six great elements; second, contemplating the impurity of the faculties; third, contemplating the impurity of the three realms. Here, contemplating the six great elements means that the bodies of sentient beings are formed of the six elements, namely the four great elements as well as the space element. The defiled consciousness of the form realm is also entirely defiled and destructible, and hence is called impure. Contemplating the six elements separately is therefore called "part by part."