Bhashya
तत्र दृष्टान्तः— स यथा लोके ऊर्णनाभिः लूताकीट एक एव प्रसिद्धः सन् स्वात्माप्रविभक्तेन तन्तुना उच्चरेत्उद्गच्छेत्; न चास्ति तस्योद्गमने स्वतःऽतिरिक्तं कार-कान्तरम्- यथा च एकरूपादेकस्मादग्नेः क्षुद्रा अल्पाःविस्फुलिङ्गाः त्रुटयः अग्न्यवयवाः व्युच्चरन्ति विविधं नानावा उच्चरन्ति- यथा इमौ दृष्टान्तौ कारकभेदाभावेऽपिप्रवृत्तिं दर्शयतः, प्राक्प्रवृत्तेश्च स्वभावत एकत्वम्- एव-मेव अस्मात् आत्मनो विज्ञानमयस्य प्राक्प्रतिबोधात्यत्स्वरूपं तस्मादित्यर्थः, सर्वे प्राणा वागादयः, सर्वेलोका भूरादयः सर्वाणि कर्मफलानि, सर्वे देवाःप्राणलोकाधिष्ठातारः अग्न्यादयः सर्वाणि भूतानि ब्रह्मादि-स्तम्बपर्यन्तानि प्राणिजातानि, सर्व एत आत्मन इत्य-स्मिन्पाठे उपाधिसंपर्कजनितप्रबुध्यमानविशेषात्मान इत्यर्थः,व्युच्चरन्ति ।यस्मादात्मनः स्थावरजङ्गमं जगदिदम् अग्नि-विस्फुलिङ्गवत् व्युच्चरत्यनिशम्, यस्मिन्नेव च प्रलीयते जल-बुद्बुदवत्, यदात्मकं च वर्तते स्थितिकाले, तस्य अस्य आ-त्मनो ब्रह्मणः, उपनिषत्-उप समीपं गमयतीति अभिधा-यकः शब्द उपनिषदित्युच्यते- शास्त्रप्रामाण्यादेतच्छब्दगतोविशेषोऽवसीयते उपनिगमयितृत्वं नाम; कासावुपनिषदि-त्याह- सत्यस्य सत्यमिति; सा हि सर्वत्र चोपनिषत् अलौ-किकार्थत्वादुर्विज्ञेयार्थेति तदर्थमाचष्टे- प्राणा वै सत्यं ते-निर्मिनुयात् अतोऽयुक्तमिति चेत्— न, शास्त्रात्; शास्त्रं संसारिणः ‘एवमेवास्मादात्मनः’ इति जगदुत्पत्त्यादि दर्शयति; तस्मात् सर्वं श्रद्धेयमिति स्यादयम् एकः पक्षः ॥
In plain words
Here's the picture: think of a spider — just one creature, nothing unusual about that — spinning itself outward on a thread that is nothing but itself. There's no separate helper involved in its spinning; it needs nothing beyond itself. Or think of a single fire, giving off countless tiny sparks in every direction — mere fragments of the fire itself. Both pictures show the same thing: things pour outward without any second agent being needed, and before any of that pouring-out begins, there is only oneness. It's exactly the same with the awareness-self: before it wakes, whatever its own nature was — that is the very source everything comes from. 'All the vital breaths' means speech and the rest; 'all the worlds' means the earth and the rest, along with everything action produces; 'all the gods' means those who preside over the vital functions and the worlds, fire chief among them; 'all beings' means every living thing there is, from Brahmā all the way down to a blade of grass. And by one reading of the text, 'all these come from the self' points to the individual selves that come into being wherever this one self meets a particular set of limiting conditions. All of this pours out of the self. Because it is from this self that the whole world, everything that moves and everything that doesn't, is forever pouring forth — like sparks from a fire — and it is into this very self that the world dissolves again, the way a bubble dissolves back into water, and it is this self's own nature that the world is made of for as long as it lasts — because of all this, this self, Brahman, is given the name 'upaniṣad,' a word that means 'that which brings you near.' Scripture itself fixes exactly what that nearness means: it is what leads a person close to the truth. So what is this upaniṣad, really? Here is the answer: 'the truth behind the truth.' Wherever this word appears, it is pointing at something beyond ordinary experience — which is exactly why it's so hard to grasp. That's why scripture spells it out directly: 'the breaths of life are real' — [the text breaks off here and picks up again] — 'you would end up fabricating something, and that can't be right,' someone might object. No — because scripture itself is the ground for this teaching: for people still caught in the cycle of rebirth, it shows, through the words 'just so, from this very self,' how the whole world comes into being. So everything here deserves to be trusted. That, at least, is one way of looking at it.
Close to the Sanskrit
Here is the illustration: just as, in the world, the spider — being just one, well known as such — comes forth, issues out, by a thread not distinct from its own self, and there is nothing else, beyond itself, that is a further factor in its emergence; and just as, from one and the same fire, tiny, minute sparks — fragments, particles of fire — fly forth variously, in many ways; just as these two illustrations show activity occurring even without any difference of agent, and show oneness of nature prior to that activity — exactly so, from this self, the vijñānamaya self, because of [its state] prior to awakening, whatever was its own nature, from that [same nature everything now proceeds] — that is the meaning: 'all the vital breaths' means speech and so on; 'all the worlds' means the earth and so on, and all the fruits of action; 'all the gods' means those presiding over the vital functions and the worlds, fire and so on; 'all beings' means the classes of living creatures from Brahmā down to a blade of grass — and, in this variant reading, 'all these [proceed] from the self' means the particular selves that become manifest through contact with limiting adjuncts — that is the meaning: [all of this] issues forth. Because, from this self, this world, moving and unmoving, issues forth unceasingly like sparks from fire, and into which alone it dissolves like a bubble in water, and whose very substance it consists of during the period of its persistence — of this very self, Brahman, the word 'upaniṣad' — being the word expressing 'it brings one near' — is used, thus called; from the authority of scripture, a particular sense belonging to this word is ascertained, namely the quality of 'leading one near.' [One asks:] 'What is this upaniṣad?' — the answer is given: 'the truth of the truth.' For this upaniṣad, everywhere, having a transcendent referent, has a meaning hard to grasp; therefore [scripture] explains that meaning: 'the vital breaths, indeed, are the truth' — [the source breaks off here and resumes] — 'would fabricate [something], hence this would be improper' — if this is the objection — no, because of scripture; scripture shows, to those bound in transmigration, by the words 'even so, from this self,' the origination of the world and so on; therefore everything here is to be trusted — this would be one position.