金剛三昧經論 2.155

2.155

論曰：此是第四述成，如其次第述彼二種不思議句。所言心者，謂入證心。無心之心入於不入，故不思議。如不異心者，釋前不思議。心本如故者，釋後不思議也。

The commentary states: This is the fourth [part], ratification, which confirms those two phrases of inconceivability in their respective order. What is called 'mind' refers to the mind that enters realization. The mind of no-mind enters into non-entering, hence it is inconceivable. 'Thusness is not different from mind' explains the former inconceivability. 'Because mind is originally thus' explains the latter inconceivability.