In this chapter, Lord Krishna has come to the topic of jñeyam.  This word jñeyam is nothing but param brahma of Upanishad.  Param Brahma is the subtlest topic of Upanishad and Bhagavat Gita.  Param Brahma is nirgunam so is not available for sensory perception.  It is all pervading.  It is only one, Ekam.  The objects in the universe are many, but the subject is only one.  It is one but appears to exist only with objects – just like the light appears only when reflected by the finger not to be present in between fingers, but it is all pervading.  It is almost impossible to conceive such a Brahman.  You have to train to understand Brahman, one of the method prescribed is to meditate upon the word Akasa or space.  Because when you meditate on akasa, you don’t see, touch, smell etc.  Even though Akasa is not perceptible, tangible, only one and not divisible.  Space may mean emptiness, but it is not empty but a positive object.  But still we are able to talk about it.  Akasa dhyānam is prescribed to understand the subtle object, similarly, try to understand Brahman, which is even more subtle.

Verse 18

It is the light of all lights.  It is said to be beyond ignorance.  It is the knowledge, the object of knowledge, and the goal of knowledge, present in the heart of all.

The brahman is same as the atma or consciousness:

• Consciousness is not a part, product, or property of an individual.
• Consciousness is an independent principle, pervading the body and making it alive.
• Consciousness is not limited by the boundaries of the body.
• Consciousness survives the fall of the body.
• Surviving consciousness is not accessible because there is no medium.

That consciousness is the existence Brahman.  In Vedanta, light is defined as that in whose presence things are known and recognized.  From that standpoint all luminaries in the sky like sun, moon, lightning are all called light, because in their presence we recognize and know the objects.  Extending this principle, every sense organ can also be known as light, because objects are known only in the presence of sense organs.  Only in the presence of our ears we know the sound.  In the presence of eyes, we see the object.  Every instrument of knowledge is light.  The ultimate light is the consciousness principle, only in the presence of consciousness everything is known. If consciousness is not there, an inert thing cannot know anything.  If the light of consciousness is not there, then the sun, moon etc. becomes useless. Therefore, consciousness is called light of light, because in the light of consciousness alone all other light reveals other objects.  The light of consciousness is unique, because only in the light of consciousness you can know darkness.  Light can’t make you know what darkness is.  Consciousness is a unique light, because it can co-exist with darkness.  This formless existence consciousness alone appears as the formed universe.  Similar to when energy is converted into matter, It becomes tangible.  Nontangible Brahman alone is the cause of the universe.  That Brahman alone is the means of knowledge; the object you know is also Brahman.  The instrument is also Brahman.  You have to discover that Brahman only in your mind.  But we have no way of recognizing the consciousness in others.  But this consciousness is in everyone and also between everyone. 

The topics of kṣetrajña and jñeyam are one and the same principle.  The consciousness obtained in from micro standpoint is kṣetrajña and the consciousness from  the standpoint of macro is jñeyam. 

Verse 19

Thus kṣetram, jñeyam and jñānam have been taught briefly.  Knowing this, my devotee becomes fit to attain My nature.

In this verse, Lord Krishna consolidates the teachings of the four topics already discussed – kṣetram, kṣetrajña, jñeyam and jñānam.  Krishna also indicated that bhakti is an important factor in knowing this knowledge.  Without devotion, jñānam does not take place.  Without devotion, he may be an expert, but the knowledge will be academic. One who get this knowledge,  he will discover the eternal freedom, which is his very nature.  Poornatvam is the benefit of this knowledge.

Verse 20:

Know both Prakriti and Purusha and purusha to be beginningless.  Know all the products and gunas to be born of prakriti.

Verse 20 to 24 Krishna teaches purusha and prakriti; this is similar to kṣetram (matter Principles) and kṣetrajña (Consciousness principle).  Vedanta takes consciousness as an independent principle.  All objects are made up visible matter principle and invisible consciousness principle.