大乘起信論別記 1.8

1.8

言「何以故」下，解意。云心法雖一，而有二門：「真如門」中，有大乘體；「生滅門」中，亦有「相用」。大乘義雖多，莫過「體相用」，故依一心顯大乘義。

From "Why is this?" downward, it explains the intent. It states that although the mind-dharma is one, it has two gates: within the gate of tathatā, there is the essence of the Mahāyāna; within the gate of arising and ceasing, there are also characteristic and function. Although the meanings of Mahāyāna are many, they do not exceed essence, characteristic, and function; therefore, relying on the One Mind, the meaning of Mahāyāna is revealed.