Avatarika
तत्र ‘इदं सर्वं यदयमात्मा’ इति प्रतिज्ञातम् ; तत्र हेतुर-भिहितः — आत्मसामान्यत्वम्, आत्मजत्वम्, आत्मप्रलयत्वंच; तस्मात् उत्पत्तिस्थितिप्रलयकालेषु प्रज्ञानव्यतिरेकेणाभावा-त् ‘प्रज्ञानं ब्रह्म’ ‘आत्मैवेदं सर्वम्’ इति प्रतिज्ञातं यत् , तत्तर्कतः साधितम् ।स्वाभाविकोऽयं प्रलय इति पौराणिकावदन्ति ।यस्तु बुद्धिपूर्वकः प्रलयः ब्रह्मविदां ब्रह्मविद्यानि-मित्तः, अयम् आत्यन्तिक इत्याचक्षते- अविद्यानिरोधद्वारेणयो भवति ; तदर्थोऽयं विशेषारम्भः—
In plain words
Earlier, the claim was made: 'all this is nothing but this very Self.' And the reason for it was given too — the Self is the one universal shared by all, everything is born from it, and everything dissolves back into it. So, because nothing exists apart from consciousness at any of the three moments — arising, persisting, or dissolving — what was claimed, that 'consciousness is Brahman' and 'the Self alone is all this,' now stands proven by reasoning as well. Followers of the Purāṇas call this kind of dissolution natural. But there is another dissolution, a deliberate one, that belongs only to those who know Brahman and comes about through that very knowledge. This one they call final — the dissolution that happens through the ending of ignorance itself. It is for the sake of explaining that final dissolution that this special discussion now begins.
Close to the Sanskrit
There, [the thesis] 'this is all, which this Self is' was asserted; there, the reason was stated — [namely] being the universal of the Self, being born from the Self, and dissolving into the Self. Therefore, because of [the world's] non-existence apart from consciousness during the times of origination, continuance, and dissolution, what was asserted, [namely] 'consciousness is Brahman' [and] 'the Self alone is all this,' has been established by reasoning. 'This dissolution is natural,' say the followers of the Purāṇas. But the dissolution that is deliberate, that has the knowledge of Brahman as its cause, [belonging] to knowers of Brahman — this they call 'final' — [namely] that which comes about through the cessation of ignorance. For the sake of that, this special undertaking [now begins]—