As mentioned in Brihadaranyaka Upanishad 4.4.20
एकधैवानुद्रष्टव्यमेतदप्रमेयं ध्रुवम् ।
विरजः पर आकाशादज आत्मा महान्ध्रुवः ॥
It should be realised in one form only, (for) It is unknowable (अप्रमेय) and eternal (ध्रुवम्). The Self is taintless, beyond the (subtle) ether, birthless, infinite and constant.
Doubt – How can Brahman is unknowable (अप्रमेय), yet is it said that one should know it ?
No, it is not a problem, because it is said in context to the worldly paramanas which are used to see other objects. As other objects doesn’t require Shabd Pramana, and can be verified by other means of knowledge, similarly this Atman is not an object of other means of Knowledge.
Secondly being everything as Atman alone , how can it be verified as another object, there cannot be Pramata-Pramana relationship here. If Scriptures explain the Meru or Swarga Loka, it can be done because it an object other than the self, but Atman as object other than the seeker cannot exist.
A relationship between what can be known and knower can only exists when there are two different objects, it cannot happen in case of Atman, where the knower and Known is same.
This belief that “I am the body” , correcting this itself is Brahman Knowledge. There is no need to know Atman using Shabd Pramana, because it is knower itself. The removing of this notion that I am body is enough to get Braham Knowledge.