成唯識論述記 4.472

4.472

述曰：自下設許有體用難。去、來識等如上所言，亦非食性；非現在故；如虛空等，以無為非食故，非現在因，雖現在者非皆是食體。是同品定有性故，得成正因。

The Commentary states: Next is the objection provisionally granting they have substance and function. Past and future consciousnesses and so forth, as stated above, also do not possess the nature of nutriment; because they are not present; like space and so forth, because unconditioned dharmas are not nutriment. As for the reason 'because they are not present', although present things are not all the substance of nutriment, because it is necessarily present in the homologous instances, it succeeds in being a valid reason.