Bhagavad Gita Verse 17, Chapter 15

uttamaha purushastvanyaha paramaatmetyudaahyataha |
yo lokatrayamaavishya bibhartyavyaya eeshvaraha || 17 ||
 
But distinct is that foremost person, spoken of as the supreme self, the imperishable lord, who enters and sustains the three worlds.
 
uttamaha : foremost
purushaha : person
tu : but
anyaha : distinct
paramaatma : supreme self
iti : in this manner
udaahyataha : spoken of
yaha : who
lokatrayam : in three worlds
aavishya : enters
bibharti : sustains
avyayaha : imperishable
eeshvaraha : lord
 
As we saw earlier, a single video game program can create an infinite variety of complex worlds in the computer. Let us now pose the question - what is the core, what is the essence of these complex worlds? What is the purest state of these worlds? We need to peel the layers of an onion, as it were. The worlds are not real, they are created by a computer program. A computer program is not real, it is a series of instructions in a computer’s memory. The instructions are not real, they are modifications of electricity. Ultimately, it is electricity that is creating and sustaining all of the worlds we see in a computer game.
 
Now, what if we pose the question - what is at the core of this world? If we get rid of all the limitations, what the purest state of this world? What is the purest state of our existence? The visible world is not real, because it is created by a play of the gunaas of Prakriti, and is perishable. Prakriti or Maaya is not real, though it is relatively imperishable, because it can be destroyed upon removal of our ignorance. What remains, is the eternal essence on which Maaya projects all its limitations or upaadhis.
 
Similarly, if we mentally remove our upaadhis or limitations, it will go something like this. Our body is a play of the gunaas, it is not real. Beyond the body are the senses and the mind, and they are comprised of gunaas and therefore not real. Beyond the senses and mind lies the eternal essence, untainted by Prakriti, which is the purest form of our “I”. In other words, when we remove all of our upaadhis or limitations, we come to the same eternal essence that is reached when we remove the upaadhis of the visible world.
 
We had seen the identity of our “I” with the eternal essence, of aatman with brahman, in the thirteenth chapter as well. That very same eternal essence, the purest brahman or shuddha brahman is referred to as the imperishable lord in this shloka by Shri Krishna. The three worlds here refer to either heaven, earth and hell, or even our waking, dreaming and sleeping states. The eternal essence enters and sustains all of them, but it is not some remote entity. It is the supreme self, the “I” residing within everyone.