成唯識論述記 3.172

3.172

述曰：心心所法有能緣相，不爾，心等應非能緣，無能緣相故，如虛空等。

The Commentary states: Dharmas of citta and caitasikas have the mark of being a cognizer; if not, citta and the like would not be cognizers, because they would lack the mark of being a cognizer, like empty space.