The thirteenth chapter, described as "Ksetra Ksetrajna Vibhaga Yoga" or "Yoga in relation to the difference between the Field and the Knower of the Field", explains the difference between Purusha and Prakriti; the world is the field of action and the individual soul is the actor; Prakriti is the cause of action and Purusha of enjoyment.
Purusha "seated in Prakriti" enjoys the Gunas born of Prakriti; the Supreme Purusha is Creator of, Abides in and Pervades Everything.
Of Purusha and Prakriti,
The Blessed Lord said:
This body, verily, is described,
And understand, O Arjuna, thou,
What is the Field, and of what kind,
Rishis have sung of this before
The Ego and the elements,
Desire, aversion, pleasure, pain,
Freedom from objects of the sense,
Detachment, absence of concern
Devotion firm to me alone,
Pursuit of knowledge of the soul,
I will tell thee now what should be known,
With hands and feet, It is everywhere,
Shining with Gunas of the sense,
Within all beings and without,
Impartible, yet it remains
It is said to be the light of lights
I have told thee briefly what is the Field,
Know Purusha and Prakriti
Of actor, action, and its means,
The Purusha in Prakriti
The highest Purusha in this frame
And he who knoweth Purusha,
By meditation, by the soul,
Others again, not knowing this,
Whatever creature that is born,
He seeth truly who doth see
Seeing the Lord in everything,
He seeth truly who doth see
All separate beings when he sees
This Soul supreme that cannot die,
As Ether by its subtlety,
A single Sun illuminates
With eye of knowledge those who see
The body is the Field: We have to understand the idea of Kurukshetra or the "Field of Action" in the same sense. It means that, so long as this body exists, or a man lives, he must necessarily perform actions.
The knower of the Field: It is the individual soul.
God, being the Supreme Soul, is the knower of all the fields or bodies.
The Field or the body consists of the unmanifest Soul, Buddhi, Ego, Mind, the five gross elements, the ten senses, and their objects. We have explained this in connection with the eight divisions of Prakriti which comprise all things.
Brahma is called neither Sat nor Asat: Sat is being, and a Sat, non- being, and the two would appear to exhaust all classification of life. But to define God, even in the most perfect manner, is to limit him; and so he is neither the one nor the other.
God has the energy of Gunas, and yet is free from them. God creates Prakriti, with its Gunas, but is not affected by its manifestations.
Purusha and Prakriti are without a source. Here we have the Yoga point of view which holds that Prakriti coexists with God from the beginning, but he is the chief creator. She is, for the most part, a spectator of his work.
Prakriti is said to be the cause of action, while Purusha is that of enjoyment. We are told the same thing in the Upanishads.