 Mind ("manas"), Ego ("ahankara") and "Sakshi", all perform different functions but that difference of functions does not mean difference in nature or essence.With regard to the word, साक्षी ("sākṣī"), used in the following verse from Shvetashvatara Upanishad,

The Varaha Upanishad (IV) refers to the "Bhumika" ('stage of development of wisdom') which is of the form of "pranava" ("Aum" or "Om") as formed of or divided into – "akāra", "ukāra", "makāra" and "ardhmātra", which is on account of the difference of "sthula" ('gross'), "sukshama" ('subtle'), "bija" ('seed' or 'causal') and "sakshi" ('witness') whose "avasthas" ('states') are – 'waking', 'dreaming', 'dream-less sleep' and 'turiya'.