Bhashya
कस्मात्पुनः कचिन्त्र प्रतिष्ठित इति, उच्यते— यस्मात्सएव भूमा अधस्तात् न तद्व्यतिरेकेणान्यद्विद्यते यस्मिन्प्रति-ष्ठितः स्यात् ।तथोपरिष्टादित्यादि समानम् ।सतिभूम्नोऽन्यस्मिन्, भूमा हि प्रतिष्ठितः स्यात् ; न तु तदस्ति ।स एव तु सर्वम् ।अतस्तस्मादसौ न कचित्प्रतिष्ठितः ।‘यत्रनान्यत्पश्यति’ इत्यधिकरणाधिकर्तव्यतानिर्देशात् स एवाध-स्तादिति च परोक्षनिर्देशात् द्रष्टुर्जीवादन्यो भूमा स्यादित्या-शङ्का कस्यचिन्मा भूदिति अथातः अनन्तरम् अहंकारादेशःअहंकारेण आदिश्यत इत्यहंकारादेशः ।द्रष्टुरनन्यत्वदर्शनार्थंभूमैव निर्दिश्यते अहंकारेण अहमेवाधस्तादित्यादिना ॥
In plain words
Why is the infinite not established anywhere else? Because nothing exists apart from it, upon which it could depend. If there were anything other than the infinite, it might rely on it—but there is no second thing. The infinite alone is all this. Therefore it has no external support. To prevent any lingering doubt that the infinite might be separate from the witnessing self—a doubt that could arise from expressions of locus and object or from indirect phrasing—the text introduces the instruction through the notion of 'I': 'Now follows the instruction through the ego.' To reveal that the seer and the infinite are one, the infinite itself is pointed out through the expression 'I': 'I indeed am below, I am above...'
Close to the Sanskrit
From what again is it not established anywhere, it is said—because that very bhūmā is below, not apart from that does anything else exist in which it might be established. Similarly above, and so on, is the same. If there were anything else than bhūmā, bhūmā would be established in it; but that does not exist. That alone is everything. Hence therefore that is not established anywhere. Lest anyone should have the doubt that from the designation of locus and object-to-be-located in 'where one sees nothing else' and from the indirect designation 'bhūmā is below', bhūmā might be other than the seer, the jīva, therefore now the instruction follows: 'the instruction through ahaṅkāra', it is instructed through ahaṅkāra, hence it is the instruction through ahaṅkāra. For the sake of showing the non-difference of the seer, bhūmā alone is pointed out through ahaṅkāra, as 'I indeed am below', etc.