Where several configurations are equally possible, one may ask why one rather than another is actual. A specifier (mukhaṣṣiṣ) is the determining basis for that preference.
فيفتقر إلى مخصص، ويدخل تحت قدرة الغير، فيكون حادثا
It would require a determiner, fall under another's power and thus be originated.
The argument moves from “a shape exists” to “why precisely this shape?” Because the Necessary Existent cannot depend on an external determiner, corporeal specification is denied of the divine reality.