法華玄義釋籤 2.73

2.73

次問宗者，宗猶尊也主也。如國無二王，何以用二法為宗耶？答意者，因果雖二而不二也。以單因單果不獨成故，有果可取因為果因，果若有因果為因果。故知能辦藉所為期、所辦藉能而顯，如臣辦王事、王能理臣，君臣相藉共經一國，異類為譬其理亦成。況下文日月綱天等譬，文意可知。又引本迹文證，具如下引。

Next, regarding the question on the central theme (zong): "theme" is like the revered one, the master. Just as a kingdom does not have two kings, why use two dharmas as the central theme? The meaning of the answer is that although cause and effect are two, they are not two. This is because a single cause or a single effect cannot be established independently: when there is an effect to be attained, the cause serves as the cause of the effect; when the effect has a cause, the effect serves as the effect of the cause. Therefore one knows that the capable agency relies on that which is accomplished as its goal, and that which is accomplished relies on the capable agency to be made apparent. It is like an official accomplishing the king's affairs, while the king is capable of governing the official: sovereign and minister rely on each other to jointly manage one kingdom. The principle also holds when using different categories as metaphors. How much more so with the similes below of the sun and moon, or the principal cord across the sky, whose textual meaning can be known! Furthermore, the textual citations of origin and traces provide proof, as cited fully below.