Bhagwad Gita - Post 190

Verse 17

But distinct is the Supreme Purusha called the highest Self, the indestructible Lord who, pervading the three worlds, sustains them.

Verse 18

As I transcend the perishable and am even higher than the imperishable, I am declared as the highest Purusha in the world and in the Vedas.

The Supreme Purusha is different from both the purusha and prakriti mentioned. He is called Paramatma, the Supreme Self. We may call the purusha of the Sankhya as a kind of self, but this is a Supreme Self which includes every other kind of self, and all selves are subsumed under this universal inclusiveness. 

In this verse (no. 18), the Supreme Lord Krishna confirms His position as Purushottama the Supreme Being of all existence by His validation of transcendence to both the kshara or perishable and akshara or imperishable.

The Brihadaranyaka Upanishad V.VI.I beginning manomayo-ayam puruso states: The resplendent Supreme Being is the ruler and lord of all governing all that exists.

Chinmaya explains in his own unique way, 

“If two trains are both moving at the same speed no movement is recognized by perceivers in both the trains. If the changes in the world of Matter --- the body, the mind and the intellect are recognized, then there must be a steady principle that illumines all these different changes. This constant factor among the Perishing is called the Imperishable.

This illumining factor gathers its status as the "Imperishable" only with reference to the "Perishable" realms. Once the perishable realms are transcended, the Imperishable amidst them Itself comes to shine forth as the Pure Infinite, which is the Purushottama. Since the Truth, Purushottama, is experienced only on transcending both the 'Perishable' and the 'Imperishable,' It is known by the term the 'Highest-Spirit' --- Purushottama. This term is used to indicate the Supreme-most Self, both by the ancient sacred volumes (Vedas), and by the poets and writers of the world.”

Love.