大乘起信論義記 2.173

2.173

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.