俱舍論疏 2.45

2.45

論「無色界繫至無色界繫」，第三釋無色也。無色界中唯有後三，要離色、欲方生彼故。色者即是五根、五境。離色欲故，彼無根境。雖不離心欲，以無根、境、五識亦無。由此唯有後三界也、即是意、意識、法界三也。

On the Śāstra's: "Bound to the formless realm... bound to the formless realm": this is the third part, explaining the formless realm. In the formless realm there are only the last three, because one must be separated from lust for form in order to be born there. "Form" refers to the five faculties and five objects. Because of detachment from lust for form, there are no faculties or objects there. Although one is not detached from lust for mind, because faculties and objects are absent, the five consciousnesses are also absent. For this reason, there are only the last three dhātus, namely the mind, mental consciousness, and dharma dhātus.