Bhagavad Gita, Chapter 15, Verse 16

There are two Purushas in the world, the Perishable and the Imperishable. All beings are the Perishable; and the Kutastha is called Imperishable.

A real, basic division is drawn here between two kinds of existence, before anything more is explained.

Naming the perishable and imperishable as two distinct categories gives Arjun a real framework to hold onto before the deeper teaching arrives. Structure comes first, detail follows.