維摩經玄疏 4.100

4.100

四、約體用明本迹者，即是法身為體，應身為用。故《金光明經》云「佛真法身，由如虛空；應物現形，如水中月」，正由虛空有實月之本體，故有一切水月之影用。今明理行合為不思議法身之理本，由此法身，故能垂不思議應用之迹；由此應用，能顯法身。故肇師云「非本無以垂迹，非迹無以顯本」，本迹雖殊，不思議一，即其義也。

4. Clarifying origin and traces in terms of substance and function: This refers to the dharma-body as substance and the response body as function. Thus the Suvarṇaprabhāsa Sūtra says: 'The Buddha's true dharma-body is just like empty space; manifesting forms in response to things is like the moon in the water.' Precisely because empty space has the fundamental substance of the real moon, there is the shadow-function of all moons in water. Now clarifying that principle and practice unite to become the origin in principle of the inconceivable dharma-body; due to this dharma-body, one is able to manifest traces of inconceivable responsive application; due to this responsive application, one is able to reveal the dharma-body. Thus Master Zhao said: 'Without the origin, there is no way to manifest traces; without traces, there is no way to reveal the origin.' Though origin and traces are distinct, inconceivably they are one—this is precisely its meaning.