From 'Brihadaranyaka Upanishad'

As long as there is duality, one sees the other, one hears
the other, one smells the other, one speaks to the other,
one thinks of the other, one knows the other; but when
for the illumined soul the all is dissolved in the Self, who
is there to be seen by whom, who is there to be smelt by
whom, who is there to be heard by whom, who is there to
be spoken to by whom, who is there to be thought of by
whom, who is there to be known by whom? Ah, Maitreyi,
my beloved, the Intelligence which reveals all by what
shall it be revealed? By whom shall the Knower be  known?
The Self is described as *not this, not that.*  It is incom-
prehensible, for it cannot be comprehended; undecaying
for it never decays; unattached, for it never attaches;
unbound, for it is never bound. By whom, O my beloved,
shall the Knower be known?