Sutra 1.1.10

10. On account of (the individual soul) going to the Self.

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-apīta)' (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 Pradhāna 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' (jīva) 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 jīva., 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' (Bṛ. 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 prāṇa alone;—from that Self the prāṇas 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 Vṛttikāra 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 Sūtra gives an additional reason.