注維摩詰經 8.73

8.73

四種性即是空種性，如前際後際空故中際亦空，若能如是知諸種性者，是為入不二法門。

The nature of the four elements is itself the nature of the space element. Just as the past and future limits are empty, so too is the middle limit empty. If one can know the natures of all elements in this way, this is entering the Dharma-door of non-duality.”