大乘起信論義記 2.127

2.127

重釋中言又心起者，牒上覺心初起之言，非謂覺時知有初相，故云無初可知。既無初相，何故說言知初相耶？釋云：言知初者，即謂無念。譬覺方時，知西即東，更無西相可知，言知西者，謂即東也。覺心之時，知初動念即本來靜，故云即無念也。

In the re-explanation, 'furthermore, as for the arising of the mind' takes up the earlier words 'awakening to the initial arising of the mind.' It does not mean that at the time of awakening one knows that there is an initial mark; hence it says 'there is no initial [mark] that can be known.' Since there is no initial mark, why does it say 'knowing the initial mark'? The explanation says: speaking of knowing the initial [mark] means thoughtlessness. Just as when realizing the directions, one knows that west is itself east and there is no mark of west that can be known; saying that one knows west means it is precisely east. When awakening to the mind, one knows that the initial stirred thought is originally quiet; hence it says 'that is thoughtlessness.'