成唯識論演祕 6.575

6.575

釋曰：謂受種子以觸為緣，後際現受果之因故，故名因受。現起報受為緣生，愛是前際果，故名果受。

The explanation is: the seed of feeling conditioned by contact is the cause of the fruit of the actual feeling of the future temporal period, hence it is called causal feeling. The presently arising retribution feeling serving as the condition that generates craving is the fruit of the prior temporal period, hence it is called fruitional feeling.