kimucyate---, yat tadajaḍarūpatvam, prakāśyavastvantarābhāvāt prakāśāntaravirahāc ca nabhovartyālokavat prakāśarūpatvād abhidhīyate bodharūpateti
What is being said is: that non-inertness, due to the absence of another object to be illuminated and the absence of another source of illumination, is called "having the nature of consciousness" because it has the nature of light, like light pervading the sky.