With the 19^th verse, Lord Krishna has completed four topics out of six topics that Arjuna wanted to know.  The four completed topics are: kṣetram, kṣetrajña, jñeyam and jñānam.

From the 20^th to 24^th verse, Lord Krishna deals with Purusha and Prakriti.  Purusha is Brahman and Prakriti as Maya.  So, this discussion is Brahman and Maya.  Purusha and Prakriti are basic principles and are anathi; We call the mixture of purusha and prakriti as Eeswara.  Eeswara existed before shristi or the origination of the universe; so, the shristi must have come from Eeswara.

The common features of Purusha and Prakriti is that they both are beginningless and the both of them together created shristi, exactly like father and mother.

Uncommon features or difference between Purusha and Prakriti are:

1. Purusha is chethana tatvam or consciousness principle and Prakriti or Maya is achethana tatvam or material or energy principle.
2. Purusha is nirvikāra tatvam – not subject to time and modification, ever changeless principle.  Prakriti is savikāram or subject to change and the influence of time.
3. Purusha is free from all attributes; nirguna tatvam; matter or Maya is endowed with all attributes or saguna tatvam.
4. Purusha has an independent existence and is satya tatvam; it is intrinsic.  The matter principle depends on consciousness principle, and it is called mithyam.

The purusha prakriti mixture called God was there before the existence of world.  We will not be able say where God was located, because before creation, even akasha or space was not there.

Nothing happens to purusha tatvam and will remain the same.  All the changes must happen to prakriti, the basic matter or energy principle.  The universe evolves from this prakriti principle.  All the products are born out of this prakriti or maya.  Initially the five basic elements were born, and later through these five elements, all other matters were born.  Our physical body is also an evolved prakriti, because the body is matter and made up of chemicals and elements.  The body is nothing but a chemical bundle and is subject to modification.  Therefore, the body is a product of prakriti.  The mind is also the product of prakriti, subject to change.

If both mind and body are born out of prakriti, then they both must be insentient. But they appear to be sentient.  That is because they are refined version of prakriti, they are able to reflect borrowed consciousness. 

Not only all the objects are born out of prakriti, all the properties of objects are also born out of prakriti.

Verse 21

Prakriti is said to be the cause in the creation of the physical and subtle bodies.  Purusha is said to be the cause in the experience of pleasure and pains.

Everything in the universe is born out of prakriti, including body mind complex.  If everything is product of prakriti, where is purusha?  Experienced world is prakriti; experienced body is prakriti; experienced mind is prakriti; If everything is product of prakriti, where is the purusha?  If you look for purusha outside, everything outside is subject to change, so it is prakriti.  If you look for purusha within the body, body is also subject to change, so it is prakriti.  Inside the mind, the thoughts are always changing, so it is also prakriti. 

Purusha is the experiencer, I, the basic conscious principle.  I am the witness of the world, witness of the body, witness of mind.  It is not an object, because all objects are subject to change and are prakriti. Purusha is boktha, the experiencer of body.  The body and the all conditions of body belong to  prakriti.  All the observed properties can belong only to the observed object and never to the observer.  The observer is always property free.

In the created world, prakriti is available as object and purusha is available as subject and purusha can never be objectified.  This is similar to how the eye can never see itself. If the eyes can never be seen, what is the proof of the existence of the eyes.  Because you don’t require proof of the existence of eyes, because every object seen is proof of the existence of eyes.  The subject does not require proof because the very search for proof proves the existence of the subject.

Verse 22

Residing in Prakrit, Purusha experiences the gunas born of prakriti.  Attachment to the gunas is the cause of birth in superior and inferior wombs.

Before creation, purusha was neither the subject nor the prakriti was object.  When did purusha get the subject status?  At that time consciousness was all pervading un-enclosed consciousness.  After the creation, consciousness was still all pervading but enclosed within the body mind complex.  The whole world becomes an object.  Purusha becomes the subject and prakriti becomes object.