釋氏要覽 1.175

1.175

凡寺院有開三門者。只有一門亦呼為三門者何也。佛地論云。大宮殿。三解脫門。為所入處。大宮殿喻法空涅槃也。三解脫門謂空門。無相門。無作門。今寺院是持戒修道。求至涅槃人居之故由三門入也。

Whenever a monastery opens three gates, or even when there is only one gate, why is it also called the 'Three Gates'? The Buddhabhūmi-śāstra states: 'The great palace has the three gates of liberation as its points of entrance.' The great palace is a metaphor for the nirvāṇa of emptiness of phenomena. The three gates of liberation refer to the gate of emptiness, the gate of signlessness, and the gate of wishlessness. Present-day monasteries are inhabited by people who uphold precepts, cultivate the Path, and seek to attain nirvāṇa; therefore, they enter through the Three Gates.