6 ideas
The individual innermost self (Atman) is not a fragment of ultimate reality but is identical to the absolute ground of all being (Brahman). Realizing this identity directly — not believing it intellectually — is the central liberating insight: 'Tat tvam asi,' you are That.
The human being is structured as concentric layers (koshas) — physical body, vital energy, mind, intellect, and bliss — each progressively subtler and each commonly mistaken for the true self. Spiritual practice is the systematic discernment of peeling back these sheaths until one rests in the unconditioned consciousness beneath them all.
Physical death destroys only the perishable sheaths, while the underlying consciousness is uncreated and indestructible; what continues is not the personality but the deeper self. Therefore fear of death rests on a category error — mistaking the changing instrument for the unchanging awareness using it.
Offered wealth, long life, and earthly pleasures by Yama, the lord of death, the boy Nachiketa rejects them all because they are impermanent and instead demands knowledge of what survives death. The lesson is that the seeker who refuses to be bought off by lesser goods is the one fit to receive the highest teaching.
The psyche contains two selves pictured as two birds on a branch: one eats the fruit (the ego absorbed in pleasure and pain) while the other simply watches without eating (the witnessing self). Suffering arises from identifying with the eating bird; freedom comes from recognizing oneself as the silent observer.
The syllable OM is treated as the audible vibration underlying all existence, with its components (A-U-M plus silence) mapping onto waking, dreaming, deep sleep, and the transcendent fourth state. Meditating on OM is a technique for using sound as a vehicle to move from surface awareness into the deepest unconditioned ground.