俱舍論記 10.113

10.113

「此成十八具足由三」者，答。具由三緣，故成十八。故《婆沙》一百三十九云「總以三緣故立十八，謂一意識相應近行有喜、憂、捨三種自性，各緣色等六種境起，故有十八。」

Regarding "This completion of eighteen is entirely through three": the answer. It is completed into eighteen entirely through three conditions. Therefore the Mahāvibhāṣā, fascicle 139, says: 'Taken together, eighteen are established through three conditions: that is, the applications associated with the single mental consciousness have three intrinsic natures—joy, grief, and equanimity—and each arises taking six objects such as visible forms as its condition; hence there are eighteen.'