Bhashya
यत्साक्षादपरोक्षाद्ब्रह्म सर्वान्तर आत्मेत्युक्तम् , तस्य स-र्वान्तरस्य स्वरूपाधिगमाय आ शाकल्यब्राह्मणात् ग्रन्थ आ-रभ्यते ।पृथिव्यादीनि ह्याकाशान्तानि भूतानि अन्तर्बहि-र्भावेन व्यवस्थितानि ; तेषां यत् बाह्यं बाह्यम् , अधिगम्या-धिगम्य निराकुर्वन् द्रष्टुः साक्षात्सर्वान्तरोऽगौण आत्मासर्वसंसारधर्मविनिर्मुक्तो दर्शयितव्य इत्यारम्भः — अथ हैनंगार्गी नामतः, वाचक्नवी वचक्रोधुहिता, पप्रच्छ ; याज्ञव-ल्क्येति होवाच ; यदिदं सर्वं पार्थिवं धातुजातम् अप्सु
In plain words
We have already been told that brahman -- seen directly, not inferred -- is the self inside everything. Everything from here forward, all the way to the Shakalya section, exists for one purpose: to show what that inner self actually is. Earth, water, fire, wind, space -- each one sits inside the next, layer within layer. Take each outer layer, examine it, set it aside for the one still deeper within it, and what is finally left, seen directly, is the self: inside everything, no metaphor, entirely untouched by anything that belongs to this changing world. That is the whole point of what comes next. So Gargi -- Vacaknavi, daughter of Vacakru -- asked Yajnavalkya: 'All this -- everything made of earth, this whole mass of it, sitting in water...'
Close to the Sanskrit
That which has been said to be brahman, directly and immediately perceived, the self within all -- for the attainment of the true nature of that self within all, the treatise begins here, extending up to the Shakalya-brahmana section. For the elements beginning with earth and ending with space are arranged in a relation of inner and outer; by successively ascertaining and setting aside whichever of them is external relative to the next, the self, directly perceived, within all, not figurative, utterly free from every property of transmigratory existence, is to be shown -- this is the purpose of the undertaking. Then Gargi -- by name; Vacaknavi, daughter of Vacakru -- questioned him. 'O Yajnavalkya,' she said, 'this whole mass of earthy matter that exists in the waters...'