俱舍論頌疏論本 18.158

18.158

善有為法名為應習，應可修習令增長故。問：「何故無為不名應習？」答：「無為無變，不可修習令增長故。又習為果，無為無果故。」非應習解脫者，謂涅槃是善是常，超一切法，故名無上。

Wholesome conditioned dharmas are called "to be cultivated", because they ought to be cultivated so that they increase. Question: "Why are unconditioned dharmas not called 'to be cultivated'?" Answer: "Because the unconditioned does not alter, and thus cannot be cultivated so as to increase. Furthermore, cultivation is for the sake of a fruit, whereas the unconditioned has no fruit." Regarding that which is not to be cultivated, namely liberation: nirvāṇa is wholesome and permanent, transcending all dharmas; therefore it is called "supreme".