The thirteenth chapter of the Bhagavad Gita is Ksetra Ksetrajna Vibhaaga Yoga. The word kshetra means the field, and the kshetrajna means the knower of the field. We can think of our material body as the field and our immortal soul as the knower of the field. In this chapter, Krishna discriminates between the physical body and the immortal soul. He explains that the physical body is temporary and perishable whereas the soul is permanent and eternal. The physical body can be destroyed but the soul can never be destroyed. The chapter then describes God, who is the Supreme Soul. All the individual souls have originated from the Supreme Soul. One who clearly understands the difference between the body, the Soul and the Supreme Soul attains the realization of Brahman.
When one realizes that the state of diversity of living things is rooted in the One, and that their manifestation is also from That, then one becomes identified with Brahman.OPEN VERSE
Being without beginning and without qualities, O son of Kunti, this immutable, supreme Self does not act nor is It affected, although existing in the body.OPEN VERSE
As the all-pervading space is not defiled, because of its subtlety, similarly the Self, present everywhere in the body, is not defiled.OPEN VERSE
As the single sun illumines this whole world, similarly, O descendant of the Bharata dynasty, the Knower of the field illumines the whole field.OPEN VERSE