大乘起信論義記 2.103

2.103

初中言離念者，離於妄念，顯無不覺也。等虛空等者，非唯無不覺之闇，乃有大智慧光明義等故也。虛空有二義，以況於本覺：一、周遍義，謂橫遍三際，竪通凡聖，故云無所不遍也。二、無差別義，謂在纏、出障性恒無二，故云法界一相也。欲明覺義，出纏相顯，故云即是如來平等法身。既是法身之覺，理非新成，故云依此法身說名本覺。無性《攝論》云「無垢無罣礙智，名為法身。」《金光明經》名大圓鏡智為法身等，皆此義也。

In the first part, "free from thoughts" means being free from deluded thoughts, revealing that there is no non-enlightenment. "Equal to empty space" and so forth: not only is there no darkness of non-enlightenment, but it also possesses the meaning of the great radiant light of wisdom and so forth. Empty space has two meanings by which it illustrates innate enlightenment: first, the meaning of all-pervasiveness, meaning that horizontally it pervades the three time periods, and vertically it extends through ordinary people and sages; therefore it says "pervading everywhere". Second, the meaning of being without distinction, meaning that within entanglement and when freed from hindrances, its nature is constantly non-dual; therefore it says "the single mark of the dharmadhātu". Desiring to illuminate the meaning of enlightenment, the mark of emerging from entanglement is revealed; therefore it says "is precisely the equal dharmakāya of the Tathāgata". Since it is the enlightenment of the dharmakāya, in principle it is not newly accomplished; therefore it says "relying on this dharmakāya, it is spoken of as innate enlightenment". As the commentary on the Mahāyānasaṃgraha by Asvabhāva says: "Stainless and unimpeded cognition is named the dharmakāya." The Suvarṇaprabhāsasūtra names the great round mirror cognition as the dharmakāya, and so on; all have this meaning.