A definition of Jnana by Shankara

The Brihadaranyaka Upanishad, verse 4.4.20, states the following:

20. It [Brahman] should be realised in one form only, (for) It is unknowable and eternal. The Self is taintless, beyond the (subtle) ether, birthless, infinite and constant.

Here we can see that the Upanishad is stating that Brahman is unknowable. So what of Self-Knowledge or knowledge of Brahman that is so often spoken about? Shankara explains this contradiction in his commentary on this verse:

The knowledge of Brahman too means only the cessation of the identification with extraneous things (such as the body). The relation of identity with It [Brahman] has not to be directly established, for it is already there. Everybody always has that identity with It, but it appears to be related to something else. Therefore the scriptures do not enjoin that identity with Brahman should be established, but that the false identification…