Atman is Brahman. Brahman is non-dual, one without a second.+

The notion of external and internal exists only within the dualistic illusion. The dualistic illusion is a product of ignorance. In reality, there is neither external nor internal.

The external and internal are created out of a single clay. That single clay is the Soul, which is present in the form of consciousness.

Ignorant wander about deluded by the dualistic illusion taking it as a reality. The Seeker has to behold the Soul, the 'Self' indivisible hidden by the dualistic illusion (I).

Sage Goudapada’s rational exposition of Advaita:~ Whatever is seen, whether external or internal, whether by the ordinary persons or yogis, is unreal.

Atman is Brahman. Brahman is non-dual, one without a second. It has no other besides it. It is destitute of difference, either external or internal. Brahman cannot be described because the description implies a distinction. Brahman cannot be distinguished from any other than It. In Brahman, there is not a distinction between substance and attribute. Sat-Chit-Ananda constitutes the very essence or Svarupa of Brahman, and not just its attributes. The Nirguna Brahman of Sage Sri, Sankara is impersonal.

Sage Sankara:~ 'VC.63 - Without knowing and examining the external world, one can’t know the Truth, as the idea that the external world exists, won't go. It can go only by an inquiry into the nature of the external world.

A Gnani knows that there's neither unity nor plurality - the world is neither one nor many. Just as a piece of rope is mistaken for a snake, the Atman is mistaken for this diverse world. Duality is an appearance and the non-dual Atman is the real truth.

The universe is not different from the Soul, which is present in the form of consciousness because the universe is made of consciousness. Consciousness alone is real and all else (the universe) is merely an illusion. Thus, the illusion is not different from consciousness, which ultimate truth or Brahman or God in truth.

The non-dual consciousness appears as the diverse universe is only an illusion. If it really became diverse then the immortal would become mortal. The dualistic thinkers who seek to prove the origination of the unborn, by that very enterprise try to make the immortal, mortal. The ultimate nature can never change - the immortal can never become mortal and vice versa. 

There's no plurality in the nature of the Soul"; "The consciousness appears to be many"; "those who are attached to creation or production or origination are in ignorance because their approach is more practical, and they are stuck with the reality of the universe (waking) take it as real. ~ Santthosh Kumaar