Bhashya
मूलम् ऐतदात्म्यम् एतत्सदात्मा यस्य सर्वस्य तत् एतदात्मतस्य भावः ऐतदात्म्यम् ।एतेन सदाख्येन आत्मना आत्म-वत् सर्वमिदं जगत् ।नान्योऽस्त्यस्यात्मासंसारी, ‘नान्यदतो-ऽस्ति द्रष्टृ नान्यदतोऽस्ति श्रोतृ’ इत्यादिश्रुत्यन्तरात् ।येनच आत्मना आत्मवत्सर्वमिदं जगत्, तदेव सदाख्यं कारणंसत्यं परमार्थसत् ।अतः स एव आत्मा जगतः प्रत्य-क्स्वरूपं सतत्त्वं याथात्म्यम्, आत्मशब्दस्य निरुपपदस्यप्रत्यगात्मनि गवादिशब्दवत् निरूढत्वात् ।अतः तत् सत्त्वमसीति हे श्वेतकेतो इत्येवं प्रत्यायितः पुत्रः आह—भूयएव मा भगवान् विज्ञापयतु, यद्वभवदुक्तं तत् संदिग्धं मम-अहन्यहनि सर्वाः प्रजाः सुषुप्तौ सत् संपद्यन्ते इत्येतत्, येनसत् संपद्य न विदुः सत्संपन्ना वयमिति ।अतः दृष्टान्तेनमां प्रत्याययत्वित्यर्थः ।एवमुक्तः तथा अस्तु सोम्य इतिह उवाच पिता ॥
In plain words
The root of everything is this inner essence. That which is pure existence is the self of all this; that is its true nature. By this self, which is pure existence, the entire world is endowed with reality. There is no other transmigrating self, as other scriptures declare: 'There is no other seer but this, no other hearer but this.' The very self by which this world is made real is the ultimate cause, the truth, the supreme reality. It is the innermost essence of the world, for the word 'Self' naturally refers to the inner consciousness, just as conventional words refer to their objects. When the son was told, 'You are that pure existence, Shvetaketu,' he replied, 'Please, father, explain this to me further. What you have said is still puzzling to me—how all these creatures merge into pure existence day after day in deep sleep, yet remain unaware that they have done so. Please make me understand this through an illustration.' The father replied, 'So be it, my dear.'
Close to the Sanskrit
The root is this state of having this as self; that which has this Sat as its self is the self of all this; that is the state of having this as self. By this self named Sat, all this world is possessed of a self. There is no other non-transmigrating self of this, because of other scriptural texts like 'There is no other seer than this, no other hearer than this.' And by which self this entire world is possessed of a self, that alone is the cause named Sat, the truth, the supreme reality. Therefore, that alone is the self, the inner essence, the true nature of the world, because the word 'Atman' without adjunct is established in the inner self like the word 'cow' etc. Therefore, 'That Sat you are, O Shvetaketu'—the son, having been thus made to understand, said: 'Let the reverend sir make me understand further. That which was said by you, that is doubtful to me—how all these creatures day after day in deep sleep merge into Sat and yet do not know "we have merged into Sat." Therefore, make me understand by an illustration.' Thus addressed, the father said, 'Let it be so, my dear.'