妙法蓮華經玄義 8.41

8.41

心是理本者，若無心，理與誰含？以初心研理，恍恍將悟，稍入相似，則證真實，是為理本。

As for the mind being the root of principle: if there were no mind, with what would principle be contained? By investigating principle with the initial mind, in a dim and nascent state one approaches awakening; gradually entering the stage of resemblance, one then verifies the real. This is being the root of principle.