成唯識論演祕 7.139

7.139

以有染淨，隱顯二位，據斯詮顯似彼苦下能緣之心無常行也。假名無常，體實常也。

Because there are defiled and pure, hidden and manifest states, based on this expression, it resembles the ākāra of impermanence of the apprehending mind under the truth of suffering. It is metaphorically named impermanent, but its substance is truly permanent.