觀無量壽佛經疏 3.53

3.53

一、從次當想水已下，總舉觀名，即是牒前生後。此明寶樹雖精，若無池水，亦未名好。一為不空世界，二為莊嚴依報，為斯義故，有此池渠觀也。

First, from "Next one should visualize the water" downward, it generally presents the name of the contemplation, which takes up what preceded and introduces what follows. This explains that although the jeweled trees are exquisite, without pond water they could not yet be called fine. First, it is so the realm is not empty; second, it is to adorn the circumstantial retribution. For these meanings, there is this contemplation of the ponds and channels.