One of the things that exercises Swami S.S.S. in his magisterial The Method of the Vedanta is the apparently paradoxical nature of the injunction to Maitreyi by Yajnavalkya in the Bhrhadaranyaka Upanishad:
The Self, my dear Maitryi, should be realised - should be heard of , reflected on, and meditated upon. By the realisation of the Self, my dear, through hearing, reflection and meditation, all this is known.(Brh.Up. II.iv.5)
The Vedas being a pramana or valid means of knowledge it is vital that it be unequivocal. Apparent conflict with other vedic statements must therefore be reconciled and considerable ingenuity is applied to that end. It is generally wrought by bringing in the contrast between higher and lower teachings. The one is from the general secular perspective immersed in avidya and the other is from the perspective of the realised sage. In one sense to enjoin dwelling on the Self is like telling someone ‘for as long as you live, you should breathe’. For other people that there is a Self comes as news and can be the beginning of the way out of samsara. The essential attitude for Swami S.S.S. is:
For the Veda will be authoritative means of knowledge if it can awaken anyone to the truth of the sole reality of the one Self, a truth inaccessible to any other mean of knowledge.
That is the faithful background out of which the commentors work and it explains the very close analysis which is applied to Yajnavalkya’s injunction.
The highest truth in the words of Swami:
When the Vedic text says There is no other seer ...but He, it means that, from the standpoint of the highest truth, the Self is not an object that can be seen. Even from the standpoint of worldly experience, one subject is never the object of the vision of another subject.
That last slightly gnomic sentence I take to mean that we can never experience what the other is experiencing as the other. The experience of that other self is opaque to us by definition.
About the question whether one could attain realisation on simply hearing - that thou art -Swami remarks that:
Sureshvara in his Brh. Vartika does give the appearance of saying that no one could attain direct and immediate intuition of the Self merely from hearing
Sankara in his Brahma Sutra commentary takes a positive view of sravana (hearing) (B.S.B. IV.i.2) and in fact in Sureshvara’s Naishkarmya Siddhi he is of the same mind. How is this to be reconciled? It seems that in the first statement he is saying that the normal path to realisation is through the hearing of the vedas and their injunctions and then passing on to manana and nididhyasana. The other uncanny alternative is that of the almost perfected person who has faith in the apoureshya (not of human) nature of the Vedas and their truth and who hearing That Thou Art gets it in its full import. I suppose that the way out of that unlikely scenario is that although this individual may have heard it before, he never really heard it.
Again Swami S.S.S. gives no example of such a one so I assume that here is an ideal limit case.