俱舍論疏 28.66

28.66

論「又諸靜慮至名無色界」，重破救也。若以觀下麁色名無色者，四靜慮中色細欲界亦應可說出離色言，是即色界應亦名無色界。

The Śāstra says: "Moreover, the dhyānas... [are also called the formless realm]," which again refutes the defense. If it is termed "formless" because one views the gross form below [as absent], then since the form in the four dhyānas is subtle compared to the desire realm, one could also apply the statement of "departure from form" [to them]; and if so, the form realm should also be called the "formless realm."