俱舍論記 1.233

1.233

「若爾無為應不名界」者。難。有為同類因，可得名為界，無為非同類，無為應非界。

"If so, the unconditioned should not be called a dhātu" is an objection. Conditioned phenomena, being homogeneous causes, can be called dhātus; but the unconditioned is not a homogeneous cause, so the unconditioned should not be a dhātu.