It is because of the unchanging witness by which we know that we are the same Self who does not change with the changing states of body and mind, we continuously remain to be the same in all the three states of waking, dreaming and sleeping and know it effortlessly to be the same Self who was yesterday, is today and will be tomorrow, this realization of the pure witness who is eternal, never was born, never will died, is the Self-realization.
~Sw. Chidananda Tirtha