In verse 6 of chapter 15, Lord gives a very important definition of Brahman, based on the upaniṣadic mantra, occurring in Kathopaniṣhad (2-2-15) as well as Mundakopaniṣhad (2-2-10), that says “The sun does not shine there, nor the moon and the stars, nor these lightnings—not to speak of this fire. He shining, everything shines after Him. By His light all this is lighted.” In this chapter Lord, as His Higher Self of Para Prakrithi says “Neither does the sun illumine there, nor the moon, nor the fire; Attaining this Brahman, one does not return to samsara again—“. implying that samsara nivritthi is complete and total when one gets established in Brahma Jnanam. The sun, moon, fire themselves cannot illumine Brahman, as Brahman is “light of all lights” (13-18) and in Its light only everything else shines. With this glorification of Brahman, Lord starts describing Brahma Swarupam to Arjuna. Later in the course of this description itself, Lord Himself will say “That light which, residing in the sun, illumines the whole world, that which is in the moon and in the fire - know that light as Mine”.(15-12)
In verses 7 to 15 Lord Krishna points out that Brahman alone appears or manifests in the form of the Jagat and Jiva in the world. Brahman has no parts and is not visible but in its manifestations as various entities it appears divided like pot space, room space etc. Although space is one entity it looks divided as room space, pot space etc. Same way Brahman manifesting in various mediums appear to be divided in them Brahman is described as sat, chit, anantham. In non-sentient matter It manifests with Sat aspect alone and in sentient bodies It manifests with both Sat and Chit aspects. So Lord declares that it is a part of Himself that manifests as the Jiva, individual soul, in each body. The Jiva enjoys the world of objects through sense organs and mind. Brahman, the pure Consciousness, never directly illumines any object but when reflected upon mind and intellect It becomes Intelligence in which the sense objects become illumined. The Jiva, using the mind along with sense organs, enjoys the sense objects such as sound, touch, taste, smell etc. It is Brahman conditioned by mind and intellect that is Jiva and this Jiva takes various bodies from time to time. These physical, mental and intellectual expressions gives the body its individual personality and together is called subtle body. At death the subtle body departs from the gross body taking with itself all faculties -senses, mind and intellect - as the wind takes away the scent from its source i.e. flowers, leaving the gross body inert as dead body. Though the Self is the nearest and comes most easily within the range of one’s consciousness, still the ignorant and deluded do not see the Self because of their complete subservience to the sense-objects. But those with the inner eye of knowledge behold the Self as entirely distinct from the body.
After describing about Brahman’s manifestation as Jiva, Lord proceeds to describe in four verses starting from verse 12, Brahman’s manifestation as Jagat, Jagat standing for achetana prapancha. He quotes only a few instances as the representative of all. Instances quoted by Him are
1) The light in the sun, the moon, and the fire in verse 12
2) Capacity in the earth to sustain life and nourish it in verse 13
3) The digestive fire that aids assimilation of food, and the subjective warmth of life in living beings, in verse 14
4) The mental faculties of knowledge, memory etc. and the authorship and content as well as the knowledge of the Vedas in verse 15.
Lord says that the very light emerging from the sun which illumines the whole world, light coming from the moon and the fire are all manifestations of Him, the Infinite Consciousness. The manifestations are different because the equipments are different just as the same electrical energy manifests in a bulb, in a fan and in a heater in different ways.
Consciousness expressed through the sun is sun-light, through the moon is moon-light and through the fuel is fire. Yet all these are manifestations of the one Brahman only. The same Consciousness while expressing through the earth manifests as the potential fertility of the soil, its life-sustaining capacity, its mineral wealth etc. The sun, the moon and the fire are the sources of energy in the world and that which gives them all the capacity to give out energy is the Consciousness only. Consciousness functioning through the moon nourishes the plant life through moon-light. Pure Consciousness that functions as Life Force manifests as digestive fire (Vaisvanara) which assimilates food. This digestive power assimilates food in four ways through masticating, swallowing, sucking and licking. The power in the digestive system that helps us to swallow the food, assimilate it and eliminate the wasteful by-products is nothing but a manifestation of the Lord as Life force. From Brahman,the Pure Consciousness alone all memory, knowledge and forgetfulness come to us. A capacity to forget is an essential requisite for acquiring new knowledge because unless the imperfect knowledge is thrown out new knowledge cannot be assimilated. This Infinite Consciousness is the one common factor that has been glorified in all the Vedas and to realize this is the fulfilment of existence. Since the very essence of Vedas is that Consciousness which is the Ultimate Reality, everything else is a projection upon it. The seeker who listens to the Vedas, who reflects upon their wisdom and who finally experiences the fulfillment of his life is also considered as nothing other than Consciousness. Lord Krishna states that one need not make any pilgrimage seeking this Divine Power for He is seated in the hearts of all only. The heart here does not mean the physiological heart but it is the peaceful joyous mind settled in tranquillity capable of concentrating on the higher inspirations like love, tolerance, charity etc.
Lord Krishna now divides the whole creation into three parts; kshara purusha, akshara purusha and uttama purusha. Kshara purusha and akshara purusha are the material aspects of creation; as matter is viewed in two states as two manifestations. One is the visible and tangible manifest matter, kshara purusha and another is the invisible and intangible unmanifest potential form of matter which is nature in its primordial state, akshara purusha, also called Maya. Kshara purusha and akshara purusha are Lord’s lower or inferior nature. Both of them together constitute what we have seen as prakrithi in chapter 13. There is a third part which is not inert, and is beyond matter in all forms and it is the knower of matter in all forms. It is the witness of matter in all forms and it is chaitanya tatvam which Lord Krishna calls uttama purusha. The word uttama means the greatest, utkr̥ṣta tama uttama. Pure Consciousness is called uttama because matter cannot exist independent of Consciousness, whereas Consciousness can exist independent of matter, and Consciousness alone lends existence to the matter. The uttama purusha of the 15th chapter is the para prakrithi of the 7th chapter, Kshetrajna or Jeyam of 13th chapter. Lord Krishna says that uttama puruṣha or puruṣhothama is His real and higher nature, His superior nature. Purushothama does not refer to any particular form of God but it refers to formless, attributeless chaitanyam, nirguna nirvishehah chaitanyam. Whoever knows this Purushothama Iswara, he becomes omniscient and totally fulfilled. This self-fulfillment is called moksha. Lord ends this chapter by telling Arjuna that this is the most secret science which He has specially imparted to Arjuna, so that Arjuna can gain self-fulfillment, knowing this.