In his bhasya to Brhadaranyaka Upanishad 4.3.21 (the Yajnavalkya – Janaka dialogue), Sankara goes to great lengths to explain what is knowledge, ‘merging with Brahman’ or ‘unity with all’, by comparing it to the deep sleep state. I will take a large part of the quote – it is well worth the read:
“This Atman is itself the light that is Pure Intelligence, and reveals everything by its own intelligence. It has been said (pars. 15 and 16) that (he is untouched by) the roaming or by whatever he sees, or enjoys, or knows in that (dream) state. And it is also proved by reasoning that the eternal nature of the self is that it is the light of Pure Intelligence.
Now an objection is being raised: if the self remains intact in its own form in the state of profound sleep, why does it not know itself as ‘I am this,’ or know all those things that are outside, as it does in the waking and dream states?
The answer is being given: Listen why it does not know. Unity is the reason.
How is that? This is explained by the text. As the intended meaning is vividly realised through an illustration, it goes on to say: As in the world a man, fully embraced by his beloved wife, both desiring each other’s company does not know anything at all, either external to himself, as, ‘This is something other than myself,’ or internal, as ‘I am this, or I am happy or miserable’- but he knows everything outside and inside when he is not embraced by her and is separated, and fails to know only during the embrace owing to the attainment of unity – so, like the example cited, does this infinite being, the individual self, who is separated (from the Supreme Self), like a lump of salt, through contact with a little of the elements (the body and organs) and enters this body and organs, like the reflection of the moon etc. in water and so forth, being fully embraced by, or unified with, the Supreme Self, his own real, natural, supremely effulgent self, and being identified with all, without the least break, not know anything at all, either external, something outside, or internal, within himself, such as, ‘I am this, or I am happy or miserable.’
You asked me why, in spite of its being the light that is Pure Intelligence, the self fails to know in the state of profound sleep. I have told you the reason – it is unity, as of a couple fully embracing each other. Incidentally it is implied that variety is the cause of particular consciousness; and the cause of that variety is, as we have said, ignorance, which brings forward something other than the self. Such being the case, when the Jiva is freed from ignorance, he attains but unity with all. Therefore, there being no such division among the factors of an action as knowledge and known, whence should particular consciousness arise, or desire manifest itself, in the natural, immutable light of the self?”
So Sankara is very clearly saying here that when the jiva is freed from ignorance – attains jnana – then he attains unity with all, which he has gone to great lengths to unambiguously compare to deep sleep, and the loss of ‘particular consciousness’.
Subsequently in 4.3.33, he continues about the calculation of the bliss arising from the knowledge of Brahman being due to desirelessness the inevitable consummation of knowledge:
“That in which the other joys, increasing step by step in multiples of hundred, merge, and which is experienced by one versed in the Vedas, is indeed the supreme bliss called Samprasada (that experienced in profound sleep); for in it one sees nothing else, hears nothing else (and so on). Hence it is infinite, and for that reason immortal; the other joys are the opposite of that.
The Vedic erudition and sinlessness are common to the other joys too. It is the difference made by the absence of desire that leads to the increase of joy a hundred times. Here it is suggested by implication that Vedic erudition, sinlessness and the absence of desire are the means of attaining the particular types of joy . . . Of these, the two factors, Vedic erudition and sinlessness, are common to the lower planes too; hence they are not regarded as means to the attainment of the succeeding kinds of joy. For this the absence of desire is understood to be the means, since it admits of degrees of renunciation. This supreme bliss is known to be the experience of the Vedic scholar who is free from desire.
Vedavyasa also says, ‘The sense pleasures of this world and the great joys of heaven are not worth one-sixteenth part of the bliss that comes of the cessation of desire’ (Mbh. XII. clxxiii. 47).
This is the state of Brahman, O Emperor, said Yajnavalkya.”
So, the supreme bliss of liberation does not come just from erudition, but from utter desirelessness. That is the jnana of Sankara, and of the Upanishads.