Shri Shankara Acharya’s Vakyavritti Verse 41
“The indirect knowledge of ‘That’ will also come to an end. How can you explain this? The answer is that the innermost consciousness in man is by nature the infinite bliss”.
In some Shrutis Brahman is pronounced to be unknowable directly by the mind. Be it so, yet its knowability is evident by reason of the fact that it is infinite bliss in its own nature.
In the state of sleep when the mind and all power of conception and perception is absent, the Self rests in its own bliss, and no want or suffering is experienced at all.
From this fact and the experience of samadhi we infer that, although Atman cannot be known by the intellect, its nature is bliss and self-evident.