大乘起信論義疏 1.72

1.72

問曰：云何得知太虛滅耶？《文殊師利問菩提論》中問此事能，「佛告：『太虛滅者，佛眼所見。』文殊白佛：『若無虛空，諸菩薩等依何住而得修行？』佛告文殊：『依法性虛空住，此為最勝。』」以此文證，定知太虛是滅法也。既言滅者，明知無常也。

Question: How is it known that the great void ceases? In the Treatise on the Inquiries of Mañjuśrī on Bodhi, this matter's capacity is questioned: "The Buddha told him: 'That the great void ceases is what is seen by the Buddha-eye.' Mañjuśrī said to the Buddha: 'If there is no space, on what do the bodhisattvas rely to abide and practice?' The Buddha told Mañjuśrī: 'Abiding by relying on dharmatā-space, this is supreme.'" By this textual proof, one certainly knows that the great void is a dharma that ceases. Since it is said to cease, it is clearly known to be impermanent.