大乘起信論義記 2.84

2.84

In the former, stating that based on the tathāgatagarbha there is the mind that arises and ceases refers to the mind that does not arise or cease. Because it is stirred into arising and ceasing by the wind of avidyā, the mind that arises and ceases is said to be based on the mind that does not arise or cease. Yet these two minds ultimately do not have two essences; it is only with regard to two aspects that mutual dependence is spoken of. Just as motionless water, blown by the wind, becomes moving water—though motion and stillness are distinct, the essence of the water is one—one may also say that based on the still water, there is its moving water. One should know that the principle here is also like this; you may reflect on it according to this standard. That is to say, the inherently pure mind is named the tathāgatagarbha; because it is stirred into arising and ceasing by the wind of avidyā, it is said that 'based on the tathāgatagarbha there is the mind that arises and ceases.' Both the Laṅkāvatārasūtra and the Śrīmālādevīsiṃhanādasūtra are identical to this explanation. This reveals that the true mind follows motion and thus undergoes arising and ceasing, not that one adduces the basis to take what is based. Because this gate contains two aspects, it can reveal the three greatnesses; therefore, it also inclusively gathers the basis into this gate.