首楞嚴義疏注經 6.295

6.295

金剛、如幻已見上文。三世如來一門超出，故云佛母。又金剛，空也。如幻，假也。佛母，中也。二、斥失。

'Vajra' and 'illusion-like' have already appeared in the text above. The tathāgatas of the three times transcend via this single gate; hence it is called 'mother of the Buddhas'. Furthermore, 'vajra' is emptiness; 'illusion-like' is provisionality; 'mother of the Buddhas' is the middle. Second, reprimanding the failure.