摩訶止觀 6.81

6.81

云何一門即是三門？一門尚是一切法，何止三耶？所以者何？觀因緣所生法是初門，一切皆初門；初門即空，一空一切空，即是第二門；此初門即假，一假一切假，即是第三門；此初門即中，一中一切中，即是第四門。初門既即是三門，三門即是一門，但奉一門為名。雖有四名，理無隔別。

How is one gate itself three gates? One gate is already all dharmas; why stop at three? Why is this? Contemplating dharmas born of causes and conditions is the first gate, and all are the first gate. The first gate is itself empty: when one is empty, all are empty, which is the second gate. This first gate is itself provisional: when one is provisional, all are provisional, which is the third gate. This first gate is itself the middle: when one is middle, all are middle, which is the fourth gate. Since the first gate is itself the three gates, and the three gates are themselves the one gate, it is merely given the name of one gate. Although there are four names, the principle has no separation.