Link:- In the first and the second verses, the Ksetra and the Ksetrajna, were described in brief The Lord reverts to the same topic, and describes these in detail under the name of prakrti (Matter) and 'Purusa' (Spirit).
prakrtim purusam caiva viddhyanhdi ubhavapi
vikaramsca gunamscaiva viddhi prakrtisambhavan
karyakaranakartrtve hetuh prakrtirucyate
purusah sukhaduhkhanam bhoktrtve heturucyate
Know that prakrti (matter) and 'Purusa' (Spirit) are both eternal and know also, that all modifications and guns (modes) are born of prakrti. Prakrti is, said to he, the cause of all activities of the body (Karya) and external and internal organs, while Purusa is said to be, the cause of experiencing pleasure and pain. 19-20
[In the third verse, Lord Krsna ordered Arjuna to hear from Him, what the Ksetra is, what it is like, what its modifications are, and whence is what. Out of these four, the first and the third were, described in the fifth and the sixth verses, respectively. The second, will be described in the twenty-sixth and twenty-seventh verses. Now, while describing 'Whence is what' He says that all modifications and guns are born of prakrti. Modifications were described, in the sixth verse. Here in this verse, He explains that guns are born of prakrti - this is something new.
From the twelfth to eighteenth verses, there is a description of the knowable (God), while here, from the nineteenth to the thirty-fourth verses, there is a description of 'Purusa' (Ksetrajna). There, all are mentioned to be within God, while here all are mentioned to be within spirit. It means, that essentially God and individual spirit, are not two, but only one.]
Prakrtim purusam caiva viddhyanadi ubhavapi:- The term, 'Prakrtim' stands for 'Primordial Matter', the cause of the entire Ksetra (universe). Seven prakrti-vikrti (Five elements, ego and cosmic intelligence) and sixteen vikrtis (ten senses, mind and five objects of senses) - all these are the evolutes of Matter and Prakrti is their cause.
The term 'Pumsam, here stands for 'Ksetrajna, which has been called a knower of the Ksetra, in the first verse of this chapter.
As the spirit, being a fraction of the Lord, is without beginning, so is Matter. But the spirit and matter, are different in other aspects. Matter is endowed with attributes, while the spirit is attributeless; Matter undergoes modifications, while the spirit, is free from modifications, Matter is the cause of the universe, while the spirit is the cause of nothing. There is a relationship of cause and effect, in Matter and its evolutes while the spirit is free from, this relationship.