HE UPANISHADS are the sole authority regarding Brahman. You will have to take the help of the six marks (Shad Lingas) in the investigation of Brahman. The Shad Lingas go to determine what the main theme of a section is and indicate clearly that the Vedantic texts treat mainly of Brahman.
The six marks are: Upakrama-Upasamhara (commencement, conclusion), Abhyasa (practice or reiteration), Apurvata (unprecedentedness), Phala (fruit), Arthavada (glorifying passage or explanatory statement) and Upapatti (illustration).
The Sruti begins, In the beginning there was Brahman (Existence alone), one only without a second. (Chh. Up. 6-2-i) and concludes, All this has its being in It; It is the True; It is the Self; and Thou art That, Tat Satyam Sa Atma Tat Tvam Asi Svetaketu. (Chh. Up. 6-8-7). Both the beginning and the end of a section refer to Brahman alone. There should be agreement between the commencement and the conclusion of a section. This constitutes Upakrama-Upasamhara Ekavakyata, one Linga or mark.
Abhyasa is the frequent repetition of 'That Thou art'. The Sage Uddalaka repeats this nine times to his son Svetaketu in order to produce a deep impression in his mind.
Apurvata - Revelation is the sole authority regarding Brahman. Brahman is knowable only through the Upanishads or the Vedas. The Sruti expressly denies other sources of knowledge. Apurvata or unprecedentedness consists in Brahman being inaccessible to any other Pramana than the Srutis. Brahman has neither colour nor taste. So It does not come within the scope of sensuous perception (Pratyaksha Pramana). It is not endowed with attributes invariably associated and so It cannot be known through inference (Anumana Pramana). It is not similar to anything known. Therefore, It cannot be known through comparison (Upamana Pramana). It can be known only through the Srutis.
Phala or fruit is Moksha or the final emancipation through the knowledge of Brahman.
Arthavada consists of explanatory statements. They explain that Brahman creates, sustains, destroys, enters into the governs the universe.
Upapatti consists of illustrations (Drishtantas) similes and analogies such as that of the clay and the pot, the thread and the cloth, the gold and the ornaments, the ocean and the waves, etc.
We have to conclude by these marks that Brahman is the main theme of the Srutis. One goes beyond grief and evil by attaining the knowledge of Brahman, just as one goes beyond fear and trouble by the knowledge, This is not a serpent, this is only a rope.'
Have a comprehensive understanding of these six marks which will help you in your investigation or enquiry of Brahman (Atma Nirnaya). Equip yourself with the four means. Practise Sravana (hearing), Manana (reflection), Nididhyasana (meditation on the Self) and attain Eternal Bliss and Immortality!