Shri Shankara Acharya’s Vakyavritti Verse 26
“The meaning of the word ‘Thou’ in the great sentence ‘That Thou Art’ is ‘the pure knowledge which is the witness’. The Self is the witness as well as the knower, because it undergoes no modifications”.
This verse is a commentary on the famous dictum ‘That Thou Art’, which is the foundation of Vedanta.
The word ‘Thou’ does not here mean the mind, but the immutable consciousness which is beyond change.
That which changes is perishable because it exists and functions in time and space.
The changeless entity is the infinite, imperishable Self which is the witness of all the changes of the mind and the body, and the word ‘Thou’ in the great sentence (mahavakya) under consideration refers to that consciousness.