15.17 And then a third Krishna speaks

उत्तमः पुरुषस्त्वन्यः परमात्मेत्युदाहृतः
यो लोकत्रयमाविश्य बिभर्त्यव्यय ईश्वरः

uttamaḥ puruṣhas tv anyaḥ paramātmety udāhṛitaḥ
yo loka-trayam āviśhya bibharty avyaya īśhvaraḥ

But there is another, the highest person, called the supreme self — who, entering the three worlds, upholds them, unchanging.

Explanation

And then a third, which is the point of the pair.

Beyond both is the puruṣottama, the highest person — who enters the three worlds and upholds them without being changed.

So the familiar two-term scheme, perishable against eternal, turns out to be incomplete. The Gita's answer is not the eternal half of a duality; it is something the duality sits inside.

Word by word

uttamaḥ: the Supreme
puruṣhaḥ: Divine Personality
tu: but
anyaḥ: besides
parama-ātmā: the Supreme Soul
iti: thus
udāhṛitaḥ: is said
yaḥ: who
loka trayam: the three worlds
āviśhya: enters
bibharti: supports
avyayaḥ: indestructible
īśhvaraḥ: the controller