金剛經纂要刊定記 2.110

2.110

為有勝能，故云極堅極利，喻般若焉。無物下。釋極堅等相，則知若有一物能壞則非極堅，若有一物不碎則非極利也。如銀鐵雖堅遇火則融，刀劍雖利斫石則缺，非極堅利也。揀餘堅利，故加極字。

Because it possesses superior capacity, it is said to be 'utterly hard and utterly sharp', which serves as a metaphor for prajñā. From 'No object...' downward explains the characteristics of being utterly hard, and so on. One then knows that if there were a single object that could destroy it, it would not be utterly hard; and if there were a single object it could not shatter, it would not be utterly sharp. Just as silver and iron, though hard, melt when encountering fire, and knives and swords, though sharp, chip when striking stone, they are not utterly hard or sharp. To distinguish it from other hard and sharp things, the word 'utterly' is added.