成唯識論演祕 2.674

2.674

有義：不能礙物名之為虛，無色等故說之為空，虛即是空，持業釋。

There is an opinion: Being unable to obstruct objects is called 'empty' (xu); because it lacks matter and so forth, it is called 'space' (kong). 'Empty' is itself 'space'—a karmadhāraya compound.