成唯識論述記 7.590

7.590

述曰：此外人難。若無心外之緣，由何而生種種分別？分別者即八識等，無外實法，此由何生？外境既成無，妄應不起難也。

The Commentary states: This is the external opponent's objection. If there are no conditions outside the mind, due to what do the various conceptual discriminations arise? Conceptual discriminations are the eight consciousnesses and so forth; if there are no external, real dharmas, from what do they arise? This is the objection that since external objects are established as non-existent, false cognition should not arise.