金剛經纂要刊定記 5.124

5.124

大因者。謂第六般若波羅蜜也。以佛有三身，法身最大，此能得故，名為大因；六中最勝，故稱第一。

"Great cause" refers to the sixth, prajñāpāramitā. Because the Buddha possesses three bodies, and the Dharma body is the greatest, and because this [pāramitā] can attain it, it is termed the "great cause"; and as it is the most supreme among the six, it is called the "foremost".