大般涅槃經義記 8.269

8.269

就後難破性了之中，文別有四：初「若是了因復何須了」以內徵外。乳中先自有其了因，何須乳外醪煖為了？此即以外破內。現見要須外事了因，明知乳內先無性了。性了既無，烏有性酪？

In the subsequent part challenging and refuting the nature-clarifier, the text has four distinct sections: First, "If it is a clarifying cause, why again is there a need to clarify it?" interrogates the external by means of the internal. If milk previously possessed its own clarifying cause, why would it need yeast and warmth external to the milk to serve as clarifiers? This uses the external to refute the internal. Seeing that external clarifying causes are indeed required, one clearly knows that inside the milk there was previously no nature-clarifier. If the nature-clarifier is absent, how could there be a nature-curd?