Vibhuti Vistara Yoga-Bhagavad Gita Chapter 10 [The Infinite Glories of the Ultimate Truth]
Krishna Said > Shaloka: 20
English
I am the Self, O Gudakesa, seated in the hearts of all creatures. I am the beginning, the middle and the end of all beings.
Purport
In this verse Arjuna is addressed as Gudakesa, which means one who has conquered the darkness of sleep. For those who are sleeping in the darkness of ignorance, it is not possible to understand how the Supreme Godhead manifests Himself in the material and spiritual worlds. Thus this address by Krishna to Arjuna is significant. Because Arjuna is above such darkness, the Personality of Godhead agrees to describe His various opulences.
Krishna first informs Arjuna that He is the Self or soul of the entire cosmic manifestation by dint of His primary expansion. Before the material creation, the Supreme Lord, by His plenary expansion, accepts the purusa incarnations, and from Him everything begins. Therefore He is atma, the soul of the mahat-tattva, the universal elements. The total material energy is not the cause of the creation, but actually the Maha-Visnu enters into the mahat-tattva, the total material energy. He is the soul. When Maha-Visnu enters into the manifested universes, He again manifests Himself as the Supersoul in each and every entity. We have experience that the personal body of the living entity exists due to the presence of the spiritual spark. Without the existence of the spiritual spark, the body cannot develop. Similarly, the material manifestation cannot develop unless the Supreme Soul of Krishna enters.
The Supreme Personality of Godhead is existing as the Supersoul in all manifested universes. A description of the three purusa-avataras is given in Srimad-Bhagavatam. “The Supreme Personality of Godhead manifests three features, as Karanodakasayi Visnu, Garbhodakasayi Visnu and Ksirodakasayi Visnu, in this material manifestation.” The Supreme Lord Krishna, the cause of all causes, lies down in the cosmic ocean as Maha-Visnu or Karanodakasayi Visnu, and therefore Krishna is the beginning of this universe, the maintainer of the universal manifestation, and the end of all the energy.