大毘盧遮那成佛經疏 13.91

13.91

言此念與虛空等不可限量，然又虛空不得為比，故復云無等也。所以然者？虛空無所有性，不能為一切眾生作大利益也。

This mindfulness is said to be equal to space and immeasurable, yet space cannot serve as a comparison; therefore it is further called 'unequalled.' Why is this so? Because space is of a nature that has nothing, and cannot bring about great benefit for all sentient beings.