成唯識論演祕 6.43

6.43

答：初得第四由未先得彼無想定，故假熏習後方得之；退由先得，故不相例。

Answer: When first attaining the fourth dhyāna, because one has not previously attained that absorption of non-conception, one must rely on cultivation through perfuming before later attaining it; regression is due to having attained it previously, so they are not analogous.