अहमात्मा गुडाकेश सर्वभूताशयस्थितः।अहमादिश्च मध्यं च भूतानामन्त एव च॥
aham ātmā guḍākeśa sarva-bhūtāśaya-sthitaḥaham ādiś ca madhyaṃ ca bhūtānām anta eva ca
Word by word
The verse says
I am the self seated in the heart of all beings, Arjuna. I am the beginning of beings, and their middle, and their end.
In plain words
Before the list of external things begins, this.
Sarva-bhūtāśaya-sthitaḥ — seated in the inner place of every being.
He puts the nearest thing first. Everything that follows is further away than this.
Where you've felt this · in health
You look everywhere for your glasses, check three rooms, and then catch sight of yourself in a mirror. The looking was genuine and careful. The thing being looked for had been travelling with the looker the entire time, which is exactly why it did not turn up in any of the rooms.
Going deeper
The first item on a list of glories is not a mountain or a god but the reader's own interior, and the placement is deliberate.
Everything that comes after this is somewhere else — a river, a season, a letter of the alphabet. Each is a thing you can turn toward. This one is the thing turning. By naming it first, the chapter establishes that the list ahead is a set of pointers rather than destinations, and that the nearest instance was never at any distance.
It also makes the long catalogue safe to read. Without this verse, forty verses of superlatives could leave somebody feeling that the divine is located in impressive things elsewhere.
What it gives you is a floor. However little of the rest of the chapter lands, the claim here is that the search has a strange property: the thing doing the searching is already the thing being searched for, which is why no amount of looking around the room has ever worked.