The primary function of the Self is to think. In order to carry out this function the Self has a mind. This mind starts out thinking by thinking of its own existence. When all doubts are eliminated the one thing which remains is the existence of the Self. The Self knows beyond all doubt that it exists.
This basic realization is the first thing about human existence that the Self can be assured of. It is the first thing that is certain in life. It is Self-evident. The Self identifies with oneself, I am I.
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The energy of the mind is the essence of life. Aristotle