Verse 6.12.3

स य एषोऽणिमैतदात्म्यमिदद्ं सर्वं तत्सत्यं स आत्मा तत्त्वमसि श्वेतकेतो इति भूय एव मा भगवान्विज्ञापयत्विति तथा सोम्येति होवाच ॥ ६.१२.३ ॥
॥ इति द्वादशः खण्डः ॥

sa ya eṣo'ṇimaitadātmyamidadṃ sarvaṃ tatsatyaṃ sa ātmā tattvamasi śvetaketo iti bhūya eva mā bhagavānvijñāpayatviti tathā somyeti hovāca || 6.12.3 ||
|| iti dvādaśaḥ khaṇḍaḥ ||

3. ‘That which is the subtlest of all is the Self of all this. It is the Truth. It is the Self. That thou art, O Śvetaketu.’ [Śvetaketu then said,] ‘Sir, please explain this to me again.’ ‘Yes, Somya, I will explain it again,’ replied his father.

Word-for-word explanation:

Saḥ yaḥ, that which; eṣaḥ, this; aṇimā, the subtlest of all; idam sarvam aitadātmyam, the Self of all this; tat satyam, that is the Truth; saḥ ātmā, that is the Self; tat, that; tvam, you; asi, are; śvetaketo iti, O Śvetaketu; bhagavān, sir; bhūyaḥ eva, again; mā, to me; vijñāpayatu iti, will you please explain; tathā, so be it; somya iti, O Somya; ha uvāca, he [the father] said. Iti dvādaśaḥ khaṇḍaḥ, here ends the twelfth section.

Commentary:

This idea is very difficult to grasp, but how much patience the father has. Now Śvetaketu has another doubt: You say that the Self exists, but it cannot be seen. You say it is aṇimā, extremely fine—finer than an atom. Yet everything has come from the Self. How then did the Self become so gross? How did it become this physical universe that we can see?

Here we must remember the principle of cause and effect. The cause becomes the effect. The cause is subtle, fine, but when it becomes the effect it becomes gross. The tiny seed becomes the vast banyan tree. The Self manifests itself as everything that exists.

The goal of life is to realize the Self. Life is useless otherwise. This is why Śvetaketu keeps bothering his father. Again and again he says, ‘Father, please explain further.’ Uddālaka is patient. The relationship between the Self and the world is quite a riddle. How the invisible Self becomes the visible world is difficult to comprehend. The example of the seed and the banyan tree helps, but it is still not clear, so Śvetaketu pesters his father for more explanation. His father then tells him to have faith. ‘Faith’ means faith in the teacher, in the scriptures, and even in oneself. Faith is what at last clears all doubts.