大乘起信論義疏 1.71

1.71

問：若無常者，何故《涅槃經》中「諸常法中，虛空為最」？答：彼經中最者，今我言法性虛空是也。何故為最者？諸佛行體皆本有故，不滅、常也。法性之理不生不滅，故湛然常，故言為最。何得言乎？佛得常中太虛為最，太虛流無，佛常具得。汝所言者，於諸明中螢虫為最。

Question: If it is impermanent, why does the Nirvāṇa Sūtra say, "Among all permanent dharmas, space is supreme"? Answer: What is supreme in that sūtra is what I now call dharmatā-space. Why is it supreme? Because the essences of the practices of all Buddhas are originally existent, unceasing, and permanent. The principle of dharmatā does not arise and does not cease; therefore, it is quiescently permanent; hence it is called supreme. How could one say that among the permanence attained by the Buddha, the great void is supreme? The great void is a flux of non-existence, whereas the Buddha fully attains permanence. What you have said is like saying that among all lights, the firefly is supreme.