The Nature of the Self
Chapter 13's distinction between the Field that acts and the Knower that only witnesses, and chapter 15's Purushottama — the highest self standing beyond both the perishable and the imperishable.
BG 13.2
This body, son of Kuntî, is called the Field; that which knoweth it is called the Knower of the Field by the Sages.
BG 13.3
Understand Me as the Knower of the Field in all Fields, O Bhârata. Wisdom as to the Field and the Knower of the Field, that in My opinion is the wisdom.
BG 13.30
He who seeth that Matter verily performeth all actions, and that the Self is actionless, he seeth.
BG 15.16
There are two Energies in this world, the destructible and the indestructible; the destructible is all beings, the unchanging is called the indestructible.
BG 15.18
Since I excel the destructible, and am more excellent also than the indestructible, in the world and in the Veda I am proclaimed the Supreme Spirit.