大乘起信論裂網疏 2.81

2.81

前明阿賴耶識，有其覺義；又明依不覺說始覺，待始覺立本覺，而始本究竟不異，故今直明覺相四種大義也。本無垢染，故名清淨。本無形相方隅分劑可得，故如虛空。本來寂照，故如明鏡。蓋但言如虛空，則無以顯其照用。但言如明鏡，則無以顯其體相。故必合言如虛空明鏡，乃可稍譬於覺相也。

Previously it was clarified that the ālayavijñāna has the meaning of awakening; it was also clarified that based on non-awakening one speaks of initial awakening, and awaiting initial awakening one establishes original awakening, yet initial and original are ultimately not different. Therefore, here it directly clarifies the four great meanings of the marks of awakening. Being originally free from defilement, it is called pure. Because originally no physical characteristics, directions, boundaries, or measures can be found, it is like space. Being originally tranquil and illuminating, it is like a bright mirror. For if one only spoke of being like space, there would be no way to reveal its illuminating function; if one only spoke of being like a bright mirror, there would be no way to reveal its essence and marks. Therefore, they must be combined to say "like space and a bright mirror," and only then can it somewhat serve as an analogy for the marks of awakening.