金光明經文句記 3.124

3.124

初、喻用助意。清水喻正、灰等喻助，以垢難去，獨水不能，灰皂助之，水方有用。

First, an analogy for the intention of using auxiliaries. Pure water illustrates the primary; ashes and the like illustrate the auxiliary. Because grime is difficult to remove, water alone cannot do it; when ashes and soap assist it, only then does the water become effective.