Verse 8.3.3

स वा एष आत्मा हृदि तस्यैतदेव निरुक्तं हृद्ययमिति तस्माद्धृदयमहरहर्वा एवंवित्स्वर्गं लोकमेति ॥ ८.३.३ ॥

sa vā eṣa ātmā hṛdi tasyaitadeva niruktaṃ hṛdyayamiti tasmāddhṛdayamaharaharvā evaṃvitsvargaṃ lokameti || 8.3.3 ||

3. The Self resides in the heart. The word hṛdayam is derived thus: hṛdi + ayam—‘it is in the heart.’ Therefore the heart is called hṛdayam. One who knows thus goes daily to the heavenly world [i.e., in his dreamless sleep he is one with Brahman].

Word-for-word explanation:

Saḥ vai eṣaḥ, it is this; ātmā hṛdi, the Self in the heart; tasya etat eva niruktam, this is the meaning of it; hṛdi + ayam iti, ‘it is in the heart’; tasmāt, therefore; hṛdayam, it is [known as] the heart; evam vit, one who knows this; ahaḥ ahaḥ vai, every day; svargam lokam eti, goes to the heavenly world.

Commentary:

Where is the Self? It is within. Normally when we think of God, we look up in the sky or we go to a temple, thinking he is there. But in reality God is within us, in our own heart. The heart is the place where we experience the Self. It is the seat of Self-realization. So here the Upaniṣad says, for this reason we worship our own heart as Brahman.

Svarga loka normally means the ‘heavenly world.’ But here it means that we go into our own Self. We become one with the Self.

The word hṛdayam means hṛdi plus ayam. Hṛdi means ‘in the heart,’ and ayam means ‘this’—that is, this Self, the Cosmic Self, which is the source of everything, is within the heart.