大乘義章 2.7

2.7

次就涅槃以說三門。涅槃之中有體相用，性淨涅槃以之為體，方便涅槃以之為相，應作涅槃以之為用。彼體寂者名曰空門，相寂之義名為無相，作用寂者名為無作。

Next, speaking of the three gates in reference to nirvāṇa. Within nirvāṇa there are essence, characteristics, and function: naturally pure nirvāṇa serves as its essence; expedient nirvāṇa serves as its characteristics; and responsive nirvāṇa serves as its function. The quiescence of that essence is called the gate of emptiness; the meaning of the quiescence of characteristics is called signlessness; the quiescence of function is called actionlessness.