“satta yA hi cit” — existence alone is consciousness 

There is no other light to illuminate self-luminous existence

After negating all that which is asat — non durable, temporary appearance of body till ignorance as wrong identifications of true self i.e., I, we arrived at the eternal existence as the true self or aham or I.

A natural question is to know this “I”, do we need a second instrument?

As we have seen earlier philosophy seeks to know the reality through metaphysics and seek to evaluate reality of the knowledge through epistemology.

The second instrument of mind is needed in case of knowing any thing that is jadam or not self luminous. We need sunlight to make moon apparent as it do not posses light of its own. But for Sun, we need no other light as it illuminates everything else with its own light.

So, Verse 23 of upadesa saram in Telugu:

unnadi teliya vEroka cittu lEdu unnadE jnanamaunu unna cittEnu.

Same verse in Sanskrit

satva bhASikA cit kvavEtarA sattayA hi cit cittayA hi aham.

There is no different consciousness to illuminate eternal existence. The existence alone is consciousness. That consciousness alone (is the knowledge of real) I am!

So the sat i.e., existence and the cit i.e., the consciousness are not two distinct things. They are one and the same represented distinctly to signify the two aspects of the ultimate reality of SELF.