It is to such a loved and loving devotee that Krishna now offers insights into two of the subtlest concepts of Upanishadic philosophy : Purusha and Prakriti, and Kshetra and Kshetrajna. Loosely, one may translate the first pair as Male and Femal; or Spirit and Nature; or Soul and Matter; or Energy and Mass. The second can be Englished with greater precision : Field and the Knower of the Field.
Purusha informs, permeates, energises, and shines through Prakriti. Prakriti is primordial, undifferentiated nature. Under the influence of Purusha, Prakriti produces the universe, the raw and the refined, teeming, variegated life of the cosmos. However, Purusha the activating agent, itself remains unaffected. Like the sky that spreads everywhere, Purusha (or Brahman or, in differentiated form, the atman) remains pure. Paradoxically, though involved in Prakriti, Purusha is the detached, supreme witness. It is not a participant.
Another way of looking at it is to describe Prakriti, in its differentiated form, as the Kshetra, the Field, the Body, the Ground of Karma's Fruits. The Kshetrajna is the knower of the Body, the atman, the Witness, the Uninvolved Participant, the Bird Watching the Bird Eating, always pure, always free, so long as it knows the truth about itself and Prakriti. The key shloka 18 hymns this truth : 'It is the light of lights, shining through darkness, it is the only knowledge worth knowing; it is the end of knowledge; it exists in everyone's heart