Verse 8.3.4

अथ य एष सम्प्रसादोऽस्माच्छरीरात्समुत्थाय परं ज्योतिरुपसम्पद्य स्वेन रूपेणाभिनिष्पद्यत एष आत्मेति होवाचैतदमृतमभयमेतद्ब्रह्मेति तस्य ह वा एतस्य ब्रह्मणो नाम सत्यमिति ॥ ८.३.४ ॥

atha ya eṣa samprasādo'smāccharīrātsamutthāya paraṃ jyotirupasampadya svena rūpeṇābhiniṣpadyata eṣa ātmeti hovācaitadamṛtamabhayametadbrahmeti tasya ha vā etasya brahmaṇo nāma satyamiti || 8.3.4 ||

4. The teacher said: ‘Then, this person, who is the embodiment of happiness, emerging from the body and attaining the highest light, assumes his real nature. This is the Self. It is immortal and also fearless. It is Brahman. Another name for Brahman is satya, Truth.’

Word-for-word explanation:

Atha, then; yaḥ eṣaḥ samprasādaḥ, this [Self] which is the embodiment of happiness [from dreamless sleep]; asmāt śarīrāt samutthāya, emerging from the body; param jyotiḥ upasampadya, attaining the highest light; svena rūpeṇa abhiniṣpadyate, he assumes his real nature; eṣaḥ ātmā, this is the Self; iti ha uvāca, he [the teacher] said; etat amṛtam, this is immortal; abhayam, fearless; etat brahma iti, this is Brahman; tasya ha vai etasya brahmaṇaḥ nāma satyam iti, this Brahman is [also] called ‘Truth.’

Commentary:

When the self leaves the body during deep sleep, it assumes its real nature. What is that nature? Does it have any form? Not exactly. It attains its real nature as light.

Knowledge is very often associated with light. We sometimes repeat the prayer, ‘Tamasaḥ mā jyotiḥ gamaya—lead us from darkness to light.’ That is to say, lead us from the darkness of ignorance to the light of knowledge.

When we are attached to the body, we are always trying to enjoy more and more sense objects. For that reason we are always disturbed and unhappy. There is no peace or calmness in us—no serenity. If you look at an image of Buddha, what do you see? His face is calm and peaceful. If you look at the face of any god or goddess you see that peace and serenity. That serenity is our very nature, but when we are attached to the body we rarely feel it. Sometimes we smile, but that smile often signifies more pain than pleasure. It is superficial. It does not come from within. Indian scriptures remind us again and again that everything is within. Strength, knowledge, joy—it is all within us.

When we know our true nature, we are no longer attached to the body, and it is nothing for us to leave it behind. Once Sarada Devi was in a state of ecstasy and saw herself outside her body. She thought to herself: ‘How can I go back to such a body?’ For people like her, leaving the body is a matter of choice. They can assume a body or reject it as they like. After a long time she persuaded herself to return to the body.

When you can overcome the delusion that you are the body, you can reject it. You then get back your true nature, as it were. You realize you are free, full of bliss, and enlightened. You are then immortal (amṛtam) and fearless (abhayām). You have conquered fear. So long as you feel you are the body, you have fear. One day you may fall sick and die—that fear is always haunting you. When you know you are Brahman, however, you know you will never, die.

The Upaniṣad says Brahman is named satya, Truth. Truth is that which was true in the past, is now true in the present, and will be true in the future. This Self is Truth itself.