大般涅槃經義記 8.149

8.149

三破性分別，十二緣法但集從緣，無有自性，所以非有；因緣相生，是故非無。非有、非無，故名為中。

Third, refuting the conceptual discrimination of inherent nature: the dharmas of the twelve links merely assemble from conditions and have no inherent nature, therefore they are non-existent; because they arise mutually through conditions, they are therefore not non-existent. Being neither existent nor non-existent, it is therefore called the middle.