The ultimate identity of the individual and the supreme soul is the great tenet of the Vedanta. 'That art thou' (Tat Twam Asi) is the first lesson of the beginner, and the last vision of the perfected mystic.
The one supreme soul alone exists; all the separate consciousnesses of individuals are but the reflection of the one soul on the multitudinous 'internal organs' which are the creation of 'ignorance' or illusion.
To reach reality we must strip off the successive veils – the waking world first (where the soul is disguised by the gross effects) and the world of dreams next (where it is disguised by the subtle effects), till we reach that of sound sleep. Here for the time the individual soul does attain its real nature, but its inherent delusion remains latent, and is still capable of being called out into actuality.
Prof. E. B. Cowell
(Source: The Indian Antiquary, A Journal Of Oriental Research VOL. IX. Published in 1880, Page 25)