The "hindrances of principle" or "noumenal hindrances，" which， paired with the 事障， or "phenomenal hindrance" constitute one of the two main types of hindrance which impede liberation according to the Sutra of Perfect Enlightenment. The noumenal hindrance is defined as the obstruction to correct awareness， the Ina-Zbility to see things as they really are; thus an Ina-Zbility to perceive the emptiness， or suchness， of things. Compared to the "phenomenal hindrance，" the noumenal hindrance is more subtle， and so is considered to be a problem for more advanced practitioners. This hindrance is analogous with the hindrance by the known 所知障， of the Consciousness-only school. See T 842.17.916b20ff.（圆觉经）.