With reference to the 'Sat' the text says, 'Learn from me the true
nature of sleep. When a man sleeps here, he becomes united with the Sat,
he is gone to his own (Self). Therefore they say he sleeps (svapiti),
because he is gone to his own (sva-apîta)' (Ch. Up. VI, 8, 1). This text
designates the soul in the state of deep sleep as having entered into,
or being merged or reabsorbed in, the Self. By reabsorption we
understand something being merged in its cause. Now the non-intelligent
Pradhâna cannot be the cause of the intelligent soul; hence the soul's
going to its Self can only mean its going to _the_, i.e. the universal,
Self. The term 'individual soul' (jîva) denotes Brahman in so far as
having an intelligent substance for its body, Brahman itself
constituting the Self; as we learn from the text referring to the
distinction of names and forms. This Brahman, thus called jîva., is in
the state of deep sleep, no less than in that of a general pralaya, free
from the investment of names and forms, and is then designated as mere
'Being' (sat); as the text says, 'he is then united with the Sat'. As
the soul is in the state of deep sleep free from the investment of name
and form, and invested by the intelligent Self only, another text says
with reference to the same state,' Embraced by the intelligent Self he
knows nothing that is without, nothing that is within' (Bri. Up. IV, 3,
21). Up to the time of final release there arise in the soul invested by
name and form the cognitions of objects different from itself. During
deep sleep the souls divest themselves of names and forms, and are
embraced by the 'Sat' only; but in the waking state they again invest
themselves with names and forms, and thus bear corresponding distinctive
names and forms. This, other scriptural texts also distinctly declare,
'When a man lying in deep sleep sees no dream whatever, he becomes one
with that prâna alone;--from that Self the prânas proceed, each towards
its place' (Kau. Up. 111,3); 'Whatever these creatures are here, whether
a lion or a wolf or a boar or a gnat or a mosquito, that they become
again' (Ch. Up. VI, 9, 3).--Hence the term 'Sat' denotes the highest
Brahman, the all-knowing highest Lord, the highest Person. Thus the
Vrittikâra also says, 'Then he becomes united with the Sat--this is
proved by (all creatures) entering into it and coming back out of it.'
And Scripture also says, 'Embraced by the intelligent Self.'--The next
Sûtra gives an additional reason.