Bhagavad Gita Verse 15, Chapter 13

bahirantashcha bhootaanaamacharam charameva cha |
sookshmatvaattadavijnyeyam doorastham chaantike cha tat || 15 ||
 
Existing outside and inside all beings, moving as well as non-moving. It is very subtle, so it is beyond comprehension. It is distant, yet it is near.
 
bahihi : outside
antaha : inside
cha : and
bhootaanaam : all beings
acharam : non-moving
charam : moving
eva : also
cha : and
sookshmatvaat : very subtle
tat : it
avijnyeyam : beyond comprehension
doorastham : distant
cha : and
antike : near
cha : and
tat : it
 
As Shri Krishna lists the indicators of the eternal essence, we may be tempted to start looking for it with our eyes, just like we look for a lost object in the house. However, we need to continuously remind ourselves that the eternal essence is neither an object nor a concept that can be grasped by the mind. Shri Krishna uses the Upanishadic style of describing the eterna essence through contradictions and paradoxes in this sequence of shlokas, and especially in the current shloka.
 
The first contradiction mentioned is that the eternal essence is both inside and outside all inert and living entities. This point cautions us against conceptualizing the eternal essence as limited by space, as if it is only available in one location and not the other. A somewhat crude analogy is mobile phone signals, which are essentially radio waves. Radio waves are all-pervasive and they penetrate all solid objects, enabling us to make phone calls from inside as well as outside buildings.
 
The second contradiction is that the eternal essence is to be known as the ultimate knowledge, yet it is beyond comprehension, due to its subtle nature. Using the radio waves analogy, we can say that none of our senses can detect radio waves. But if we know how to build a device that can access radio waves, we can harness their power to our advantage. Similarly, the eternal essence remains beyond the comprehension of those who have not purified their mind. But for those who have purified their mind, and followed a systematic method of enquiry under the guidance of scriptures and a teacher, it is ever accessible.
 
This leads us to the third contradiction. For those who have enquired about the nature of the eternal essence systematically, it is immediately available at all times as the self, the “I” within us. But for those who are ignorant, it is far away. Shankaraachaarya says that it is unattainable even in millions of years for such people. Whenever we focus on names and forms, we lose sight of the self. But when we remove the upaadhis of names and forms, we come back to the self, the “I”, that is behind all the names and forms, just like the movie screen.
 
Note that any time we use analogies such as radio waves, we are trying to conceptualize the eternal essence which is beyond all conception. We need to consider such analogies as helpful pointers, and nothing more.