大乘起信論義記 2.127

2.127

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.'