成唯識論述記 2.239

2.239

述曰：一切未得應永不生，已失之法更不應生。此中通說，未得之法及易界地等已捨之法，應永不生；無能生得故；如龜毛等。

The Commentary states: All that has not yet been acquired should never arise, and phenomena that have already been lost should never arise again. In this general statement, phenomena not yet acquired and phenomena already abandoned through changing realms, planes, and so forth should never arise; because there is no prāpti capable of generating them; like a tortoise's hair and so forth.