pūrvāparakoṭidvayaparihārasthitalakṣaṇatvān nityatvasya, parasparaparihārasthitalakṣaṇayoścaikanirākaraṇasyāparasadbhāvanāntarīyakatvādityuktam
Since eternality is characterized by the elimination of both prior and posterior endpoints, and since between two mutually exclusive characteristics, the negation of one necessarily implies the existence of the other - this has [already] been stated.