起信論疏筆削記 6.28

6.28

標果望因者，諸佛即果，本即是因。以諸如來本所修行因地之時，無別所乘之法，唯以此心為其所乘，而至究竟。

"Indicating the fruit looking back to the cause": all Buddhas are the fruit, and "originally" is precisely the cause. Because at the time of the causal stage originally cultivated by all Tathāgatas, there was no other dharma that was ridden, and they used only this mind as that which was ridden, thereby reaching the ultimate.