成唯識論述記 2.935

2.935

述曰：心性者，真如也。真如無為非心之因，亦非種子能有果法，如虛空等。故非有漏心性是無漏，名本性淨也。

The Commentary states: The 'nature of mind' is tathatā. Tathatā is unconditioned and is not the cause of mind, nor is it a seed capable of producing fruit dharmas, like space and the like. Therefore it is not that the nature of sāsrava mind is anāsrava and thereby called fundamentally pure.