大乘百法明門論疏 1.25

1.25

解云：「何故第五明其無為？無為之法，相難了知，若不約法以明，何能顯示？故能依色、心、心所有法、不相應行四法之上顯示無為，是故第五明無為法。」

The commentary states: "Why are asaṃskṛta-dharmas explained fifth? The characteristics of unconditioned dharmas are difficult to understand; if they are not clarified with reference to dharmas, how could they be revealed? Therefore, unconditioned dharmas are revealed in dependence upon the four dharmas: rūpa, citta, caitta-dharmas, and viprayukta-saṃskāras. For this reason, asaṃskṛta-dharmas are explained fifth."