俱舍論疏 28.38

28.38

論「即此四根本至立無色名」，述同異也。此四無色，上三近分不緣色故，名除色想。空處近分以緣色故，非除色想。然四根本、四色近分，雖緣色不同，皆無色故，同名無色。

The Śāstra says: "These four fundamental [...] are established with the name 'formless.'" This states the similarities and differences. Among these four formless attainments, because the upper three preparatory stages do not take material form as intentional object, they are called 'eliminating the ideation of material form.' The preparatory stage of the sphere of infinite space, because it takes material form as intentional object, is not 'eliminating the ideation of material form.' However, although the four fundamental attainments and the four preparatory stages differ in their taking or not taking material form as intentional object, because they all lack material form, they are equally called 'formless.'