大乘起信論義記 2.173

2.173

前中以空及鏡皆有四義故，取之為喻：一、空鏡，謂離一切外物之體。二、不空鏡，謂鏡體不無，能現萬象。三、淨鏡，謂磨治離垢。四、受用鏡，置之高臺，須者受用。四中前二自性淨，後二離垢淨。又初二就因隱時說，後二就果顯時說。前中約空不空為二，後中約體用為二，又初二體，後二相，故云覺體相也。又初一及第三有空義，第二第四有鏡義，故舉二喻。

In the preceding [segment], because empty space and a mirror both possess four meanings, they are taken as similes: 1. An empty mirror, meaning it is devoid of the substance of all external objects. 2. A non-empty mirror, meaning that the substance of the mirror is not non-existent, being able to manifest the myriad images. 3. A pure mirror, meaning it has been polished and cleansed to be free of defilement. 4. A usable mirror, which, placed upon a high terrace, is used by those who need it. Among the four, the first two are purity by own-nature, and the latter two are purity free of defilement. Furthermore, the first two are spoken of when it is hidden in the cause, and the latter two are spoken of when it is revealed in the fruit. In the former, it is divided into two based on empty and non-empty; in the latter, it is divided into two based on substance and function. Furthermore, the first two are substance and the latter two are marks; hence it says 'the substance and marks of enlightenment.' Also, the first and third possess the meaning of emptiness, while the second and fourth possess the meaning of a mirror; therefore, two similes are given.