大乘起信論義記 3.385

3.385

答意云：只由內盡妄想心故，能外廣知也。於中有三：初、立正理；次、舉非顯失；後、舉是彰得。前中境雖無邊，不出一心，既證心源，何不能了？即心之境離於妄念故，盡想念方始能知，故云一切境界等也。

The meaning of the answer is: precisely because the deluded thinking mind is exhausted internally, one is able to know extensively externally. Within this there are three: first, establishing the correct principle; next, citing error to reveal fault; finally, citing truth to show attainment. In the first: although spheres are limitless, they do not go beyond the One Mind; having realized the source of the mind, how could one not fully understand them? The sphere that is identical with the mind is free from deluded thought; hence, only when thought and ideation are exhausted can one begin to know them. Therefore it says: 'All spheres,' etc.