The Sole Witness — Purusha as Unconditioned Awareness
Patanjali's radical metaphysics: the Purusha (pure consciousness) is the sole witness of all mental activity — unchanging, unaffected, eternally present, the foundation on which all experience rests without itself being touched by any experience.
Sutra 4.18 establishes the foundational metaphysical claim of the Yoga Sutras: 'sada jnatash chitta vrittayas tat prabhoh purushasya aparinaminah' — the modifications of the mind are always known to the Purusha, who is their Lord and who is unchanging. The Purusha never forgets a single mental event, is aware of every thought that arises, witnesses every sensation and emotion — and is entirely unchanged by all of it. This paradoxical combination of total awareness and total non-involvement is the nature of pure consciousness as Patanjali describes it. It is always watching but never participating; always present but never conditioned; always aware but never identified. To recognize oneself as this unchanging witness — rather than as the thoughts and experiences witnessed — is the experiential content of Kaivalya.
The modifications of the mind are always known to the Purusha, who is their Lord and is unchanging.
Philosopher Thomas Nagel's 'view from nowhere' — the concept of a perspective that could observe all perspectives without being any of them — approximates Patanjali's description of Purusha, though Nagel presents it as a philosophical ideal whereas Patanjali describes it as the always-present reality of consciousness itself.
Yoga Sutras of Patanjali