Principal Upanishad · Atharvaveda
Mandukya Upanishad 7
Turiya — the fourth, described entirely by negation
नान्तःप्रज्ञं न बहिष्प्रज्ञं नोभयतःप्रज्ञं... प्रपञ्चोपशमं शान्तं शिवमद्वैतं चतुर्थं मन्यन्ते स आत्मा स विज्ञेयः॥
Not inwardly cognitive, not outwardly cognitive, not cognitive both ways — It is consciousness itself. It is the Self. It is to be known.
What this verse points to
The famous seventh verse, describing the fourth state by ruling out everything else: not knowing outward, not knowing inward, not a mass of knowing. Every positive description is withdrawn as soon as it is offered.
This is deliberate method, not evasion. Anything the mind can form a picture of is an object, and the verse is pointing at what is never an object.
Sit with it
For one minute, decline every description of your experience that arises. Notice what does not go away when the descriptions do.
About the Mandukya Upanishad
The shortest principal Upanishad — twelve prose verses — analysing consciousness through the syllable AUM and four states: waking, dreaming, deep sleep, and the fourth (turiya). Gaudapada's karika on this text, and Shankara's commentary on both, became the cornerstone of Advaita Vedanta.
References to this text are given as verse — here, 7.