俱舍論疏 8.29

8.29

論「有色界生此二劣故」，答也。以欲、色界此二劣故依色而轉，無色此二強故相依。

The Śāstra says: "Because in births in realms possessing form, these two are weak." This is the reply: Because these two are weak in the desire and form realms, they rely on form to operate; in the formless realm, these two are strong, so they mutually rely on each other.