成唯識論述記 2.523

2.523

述曰：下申正義也。若諸無為非實是有，何故經中及此宗內，說虛空等以為無為？答：此所說略有二種。

The Commentary states: Below, the correct doctrine is set forth. If unconditioned phenomena are not real existents, why do the sūtras and our own school speak of space and the like as unconditioned? Answer: What is spoken of here is briefly of two kinds.