成唯識論述記 2.521

2.521

述曰：諸無為法非定實有；無因果故；如兔角等。他部無為無有因果，體是因果，而無因果也。因果即是六因、五果，為離繫果時，非六因所得；為能作因等時，不得五果故。自宗無為非異心等，故無過失。

The Commentary states: The unconditioned phenomena are not definitely real existents; because they lack cause and effect; like a hare's horns and the like. The unconditioned of other schools lacks cause and effect; their substance is [claimed to be] cause and effect, yet they lack cause and effect. Cause and effect refers to the six causes and five fruits: when regarded as the fruit of disjunction, it is not obtained by the six causes; when regarded as an efficient cause and so forth, it does not obtain the five fruits. The unconditioned of our own school is not different from citta and the like, and hence there is no fallacy.