Consciousness (ultimate truth or Brahman) is one without the second, the one is not in the sense half or two, but the one that remains forever One, without the second. The consciousness is all pervading. There is no place where consciousness is not.
Consciousness is in everyone, consciousness is in everything .consciousness is one behind many. Consciousness alone is. It means the universe and its contents are the visible forms of consciousness. And consciousness, in turn, is the invisible form of the universe, which appears to the mind.
Since people, all are not aware of the fact that, they are an experience within another experience (waking or dream). The mind is an experience, which contains everyone and everything and the universe. Since one limits the mind to the physical entity, he forgets the fact that, his body and the universe is an experience, within waking or dream experience. It is impossible to unfold the mystery of his true existence without realizing the fact that, the self is not physical but the self is consciousness.
If the self is not physical then it is erroneous to judge and conclude the truth on the base of the physical self or ego. The ego (waking entity) is false self within the false experience (waking). Thus the truth has to be judged on the base of the true self, which is formless consciousness which is not an individual but it pervades all the three states as their formless substance and witness.
Until and unless one rectifies the reasoning base from the form (ego) to formless (soul or consciousness), the ultimate truth or Brahman will not be revealed.
Sage Sri, Sankara says in VC :~ 56. Neither by Yoga, nor by Sankhya, nor by work, nor by learning, but by the realization of one's identity with Brahman is Liberation possible, and by no other means.
58. Loud speech consisting of a shower of words, the skill in expounding the Scriptures, and likewise erudition - these merely bring on a little personal enjoyment to the scholar but are no good for Liberation.
59. The study of the Scriptures is useless so long as the highest Truth is unknown, and it is equally useless when the highest Truth has already been known.
60. The Scriptures consisting of many words are a dense forest which merely causes the mind to ramble. Hence men of wisdom should earnestly set about knowing the true nature of the Self.
61. For one who has been bitten by the serpent of Ignorance, the only remedy is the knowledge of Brahman. Of what avail are the Vedas and (other) Scriptures, Mantras (sacred formulae) and medicines to such a one?
62. A disease does not leave off if one simply utters the name of the medicine, without taking it; (similarly) without direct realization one cannot be liberated by the mere utterance of the word Brahman.
63. Without causing the objective universe to vanish and without knowing the truth of the Self, how is one to achieve Liberation by the mere utterance of the word Brahman? — It would result merely in an effort of speech.
65. As a treasure hidden underground requires (for its extraction) competent instruction, excavation, the removal of stones and other such things lying above it and (finally) grasping, but never comes out by being (merely) called out by name, so the transparent Truth of the self, which is hidden by Maya and its effects, is to be attained through the instructions of a knower of Brahman, followed by reflection, meditation and so forth, but not through perverted arguments.
66. Therefore the wise should, as in the case of disease and the like, personally strive by all the means in their power to be free from the bondage of repeated births and deaths.
67. The question that thou hast asked today is excellent, approved by those versed in the Scriptures, aphoristic, pregnant with meaning and fit to be known by the seekers after Liberation.
That is why Sri Sankara, indicated in Bhaja Govindam says: - [Jnana Viheena Sarva Mathena Bajathi na Muktim janma Shatena] - one without knowledge does not obtain liberation even in a hundred births, no matter which religious faith he follows.
Thus it proves the religion is not the means to self-realization.
When Upanishad itself declares:- sarvam khalvidam brahma - all this (universe) is verily Brahman. By following back all of the relative appearances in the world, we eventually return to that from which it is all manifest – the non-dual reality [ Chandogya Upanishad].
Then it is no use going a roundabout way, trace the Brahman which is the formless substance and witness of the universe, which is in the form of mind. By tracing the source of the mind or universe one will be able to realize the Brahman.
Thus, self-knowledge or Brahma Gnana or Atma Gnana is meant only for those who have intense urge, and courage to accept the truth with humility and reject the untruth. Since people start comparing with their scriptural knowledge, it becomes impossible to assimilate and realize the non-dual truth. Therefore, there is no need to convince anyone other than our own-selves to get the firm conviction.