大乘起信論別記 1.76

1.76

言「水非動性」者，今雖非靜，而此動若不由自性故動，但是隨他而動。心亦如是，隨緣而動，不由自性故動，故言非動性也。

The statement "water does not have the nature of motion" means that although it is not still at present, if this motion does not move out of its own nature, it moves only by following something else. The mind is also like this: moving according to conditions, it does not move out of its own nature; hence it is said not to have the nature of motion.