Upanishads explore the "subject" in us and inform that the subject in us is not the mind, body or intellect.
According to the Upanishad thought.
mind, body and intellect are also objects. The ultimate subject in us is the essential factor which makes body function, make the mind feel and make the intellect think. This essential factor is also called as God.
The aim of human birth is supposed to be to realise that we are that God.