成唯識論述記 2.258

2.258

述曰：以得無實故，非得亦無；得有用而尚無，非得無用如何有？非得非有；得與非得相返立故；如得。

The Commentary states: Because prāpti is not real, aprāpti is also non-existent. When prāpti has a function and yet does not exist, how could aprāpti, which lacks a function, exist? Aprāpti is non-existent; because it is established in opposition to prāpti; like prāpti.