What is Unmanifest? It is Brahman without attributes, nirguṇa, without dyads, triads, unchangeable and hence it is Pure and Absolute.

Bṛhadāraṇyaka Upaniṣad explains this (III.8). Rishi Yājñavalkya says to Gārgī, “Un-manifest pervades above the heaven and below the earth and the place between heaven and earth (antarikṣa). It is in the form all that exited, existing and all that is going to exist (past, present and future). Those (possessing sacred knowledge and spiritually wise – Self-realized persons) who know Brahman  say (they only say; they do not describe) It is neither gross nor subtle, neither short or long, not red complexioned, not silky, not shadow, not darkness, not air, not ākāśa, not attached to anything (asaṅga), not taste, not smell; It is without eyes, without ears, without speech, without mind, non-luminous, without prāṇa, without mouth, immeasurable, without interior or exterior. It does not eat anything, nor It is eaten by anybody. Because of This (akṣara – imperishable), sun and moon are in their respective places. Because of This, moments, days and nights, fortnights, months, seasons, years (all related to time factor) do their duties perfectly. Rivers flow because of It. Only because of It, men praise those who give charity, gods and ancestors depend upon yajña sacrifices,  

“Gārgī! Without knowing It, there is no use of performing yajña-s and other sacrifices or undertake austerities. All such acts go in vain. The one who dies without knowing It (Brahman) transmigrates (rebirth) and the one who dies after knowing It is not born again.” {Kena Upaniṣad explains this thus: “If one realizes Brahman in this life and understands the Truth (knowledge about Brahman), is not born again. Truth is realizing Its presence in every being”. It says that without understanding Brahman, however hard one performs various acts, it could only be waste of time, leading to miseries of transmigration and also pains and pleasures in this birth (dyad called māyā). However, some effects will be there for performing rituals without vanity. This will take him only to heaven and he has to be born again. In other words, unless Brahman is realized, transmigration will continue.}

Yājñavalkya further says, “Gārgī! It is never seen*, but It sees everything (It only witnesses; soul only witnesses and does not cause any action; all actions happen only in Prakṛti/māyā). It cannot be heard, but It hears every sound (subtle sound of mantras are heard by It). It cannot be conceived through mind, yet It is thought itself. It can never be known, but It knows everything (because It perpetually witnesses all the actions). There is no other witness other than This, no hearer other than This, no thinker other than This and no knower other than This. Gārgī! It is this unmanifest that pervades everywhere.”

*It refers to Brahman. Brahman cannot be seen, as It is devoid of shapes and forms. But It can still be seen says Kaṭha Upaniṣad (I.iii.12). “The Self is hidden in all beings, but not noticed, as It is highly subtle. Those with single pointed intellect still see It.” Upaniṣad uses the words सूक्ष्मया बुद्ध्या sūkṣmayā buddhyā which mean subtle nonmaterial intellect. That is why, Kṛṣṇa gave divine eyes to Arjuna (Bhagavad Gītā XI. 8). “Arjuna! You are incapable of seeing me with your biological eyes. Therefore, I give you divine vision. With this, behold my yogic form.” It implies that Brahman (referred as It, because It is neither masculine, nor feminine nor eunuch) can be realized only through intellect (buddhi). Kaṭha Upaniṣad (I.iii.13) further explains this. “A wise person dissolves his sensory organs into the mind, mind into his intellect (discussed above), his intellect into hiraṇyagarbha** (the individual soul invested with the sūkṣma-śarira or subtle body) and the individual soul into Brahman.”  

** Vedānta Paribhāsā a 17th century scripture explains hiraṇyagarbha. It says “Hiraṇyagarbha is the first soul to be born and is different from Brahma, Viṣṇu and Śiva.”  The subtle body consisting of the five vital forces, the mind, the intellect and the ten organs is produced from the five basic elements.  This paves the way for the soul to experience the result of actions or in other words it causes karma-s. The subtle body is of two kinds, superior and inferior.  The superior one is the subtle body of hiraṇyagarbha and the inferior is the subtle body of living beings.  The subtle body of hiraṇyagarbha is called as mahat or the cosmic intellect and the subtle body of living beings is called ego.

In order to get divine vision, we have to do sādhana (practice to achieve the goal). Sādhana is the right combination of mantra, breathing, mind, intellect and ultimately consciousness.