肇論新疏 3.246

3.246

通敘前理。窮理盡性，語出《周易》。彼云：「窮理盡性以至於命。」理其然者，許可其理。

This broadly recaps the preceding principle. "Fathoming principle and exhausting nature" comes from the Book of Changes. That work states: "Fathoming principle and exhausting nature so as to arrive at destiny." "In principle it is indeed so" means assenting to this principle.