སངས་རྒྱས་ཕལ་པ་ཆ་ཞས་བྱ་བ་ཤན་ཏ་རྒྱས་པ་ཆན་པའ་མད notes.1.87

The Ten Bhūmis; The Chapter on the Scale of Life; The Dwellings of Bodhisattvas; The Stem Array; The Stem Array notes.1.87

87. According to Vasubandhu, this means that, like space, they cannot be stained by anything in the world.