Svāpyayāt स्वाप्ययात् (I.i.9)
Sva – self; apyaya – entering into (merging into).
Because, Brahman alone is the Reality, as mentioned in the previous aphorism, this sūtra says that the individual consciousness merges into the Self during deep sleep state. There are three normal stages of consciousness – active state, dream state and deep sleep state. In active state, the mind is active and is fully conscious. In the dream state, though the body is rested, mind is still active and remains in the state of semi conscious state. In the deep sleep state, the mind is also rested along with the body and this is the state, where individual consciousness remains (merging for a sojourn or merging for a brief period) with Supreme Consciousness. In other words, individual consciousness enters into or merges with Supreme Consciousness for a brief period only to identify itself as an individual consciousness (identifying itself with individual body). Why individual consciousness sojourns with Supreme Consciousness during deep sleep state and not with any other matter such as Prakṛti? This is because there is nothing else except Brahman to resort to; from whom everything originates and unto whom, everything dissolves.
(In reality, there is no such thing as individual self; everything is Self only. The differentiation between self and Self is often used only to differentiate between individual consciousness and Supreme Consciousness. It is purely for the purpose of understanding the difference between macrocosm and microcosm. What is the difference between the water contained in a cup and a tank? Water is the same in both the containers, but the difference is in the type of vessels used. Brahman is the same, but only the outer shapes and forms differ such as trees, animals, human beings, etc. Bṛhadāraṇyaka Upaniṣad (IV.iii.21) says, “This is the form of Brahman – free from desires and evils and is fearless. When the self embraces the Self, he (embodied person) does not know anything at all, be it internal or external.”)
Chāndogya Upaniṣad (VI.viii.1) explains about deep sleep state. “O! Śvetaketu, I will now explain about deep sleep state. When a person is sleeping, he becomes one with the Self (Upaniṣad uses the word “Sat” to mean the Self or Brahman) and he attains his real Self.” This saying of Upaniṣad explains many intricate things. Let us for the moment forget what has been explained in the previous paragraph in parenthesis. Let us assume for a moment that self and Self are different. Then, what is the closest part of a man that resembles the Self? Undoubtedly it is the mind. Mind is not either self or Self. But why mind is said to be closer to the Self? Because, only in the individual mind, Self can be realized (which already exists). Let us assume that there is a big playground. When there are no players, playground is pure filled with pure air and light. When there is a match in the playground, it is filled with a lot of players, spectators, sound, waste, etc. The purity of the ground is lost. Once the match is over and the ground is cleaned, the ground again becomes pure. Play ground is our mind and the players, spectators, etc, are various thought processes that make the mind afflicted. Pure air and light can be compared to Brahman. Though players and spectators are present in the ground, light and air also exist. Light and air exist all the time, without disruption. In the same way, Brahman exists in all the places, whether it is good or bad. When players and spectators leave the ground, we are able to feel the light and air in the ground; otherwise, we would have concentrated only on the players and the game played by them. This is the exact description of Brahman. Brahman, the Self and Paramātaman are all same. It is present everywhere and only thing is that we do not realize it*.
What is the benefit of merging with Brahman? What we experience during our deep sleep state is the benefit. In our deep sleep state, nothing bothers us, no worries, no desires, no attachments, etc because our mind is totally rested. This is the state of mind during our deep sleep state. Suppose, we are able to maintain this state of mind during our active state, what could be the result? The result will be on par with our deep sleep state; however the only difference being is that we are conscious during our active state and not conscious during our deep sleep state. Thus, we can understand that it is only our mind which is responsible for perceiving duality. Mind never acts on its own, but acts through sensory organs. In the playground, light and air are present along with the players. But we concentrate only on the players and the game and not on the light and air that exists in the playground all the time. Why we concentrate on the players instead of air and light? This is the attraction towards the material world and is caused by sensory organs due to the inducement of mind. Mind by default is attracted to the material world, hence mind needs to be controlled though will power.
Whether it is creation or sustenance or destruction, everywhere Brahman alone prevails. At the same time, Brahman creates, sustains and destroys (death); He is in the form of soul of sentient and insentient beings. The difference between sentient and insentient is the presence and absence of consciousness. Though, He is the cause of all the individual bodies, He does not cause any action in any of the individuals. Actions are due to karma, which is not the subject matter at this point of time.
Therefore, Brahman alone is the cause of the universe, from whom everything originates and unto Him, everything dissolves. Thus Brahman is omnipresent. Various shapes, forms and names are only due to two factors. One is māyā and the second is lack of adequate spiritual knowledge.
*Brahma Sūtra never refers Brahman as He. It refers Brahman only as It. Brahma Sūtra lays a strong foundation for knowing the Self. Hence it dwells in preliminaries elaborately. In spiritual path, foundation is extremely important. When higher spiritual knowledge is imparted, one cannot obviously ask his Guru, what is the difference between the Self and self.