Bhashya
इदं च नकारद्वयं वीप्साव्याप्त्यर्थम् ; यद्यत्प्राप्तं तत्तन्निषिध्यते; तथा च सति अनिर्दिष्टशङ्का ब्रह्मणः परिहृता भवति ; अन्यथा हि नकारद्वयेन प्रकृतद्वयप्रतिषेधे, यदन्यत् प्रकृतात्प्रतिषिद्धद्वयात् ब्रह्म, तन्न निर्दिष्टम्, कीदृशं नु खलु- इत्याशङ्का न निवर्तिष्यते; तथा च अनर्थकश्च स निर्देशः, पुरुषस्य विविदिषाया अनिवर्तकत्वात्; 'ब्रह्म ज्ञापयिष्यामि' इति च वाक्यम् अपरिसमाप्तार्थं स्यात् ।यदा तु सर्वदिक्कालादिविविदिषा निवर्तिता स्यात् सर्वोपाधिनिराकरणद्वारेण, तदा सैन्धवघनवत् एकरसं प्रज्ञानघनम् अनन्तरमबाह्यं सत्यस्य सत्यम् अहं ब्रह्म अस्मीति सर्वतो निवर्तते विविदिषा, आत्मन्येवावस्थिता प्रज्ञा भवति ।तस्मात् वीप्सार्थं नेति नेतीति नकारद्वयम् ।ननु महता
In plain words
This doubling of 'not' isn't accidental — it's there to cover everything, to reach in every direction: whatever gets proposed, that gets ruled out, and whatever else gets proposed after that, that too gets ruled out. That's what closes off the nagging doubt that Brahman has been left unpointed-to. If the two 'nots' only ruled out the two things actually named, then anything Brahman might be beyond those two specific things would remain unaddressed — the question 'well, what kind of thing is it, then?' would never actually go away. And in that case the whole teaching would be empty, because it would never lay the seeker's need to know to rest. Even the promise 'I will reveal Brahman to you' would be left hanging, unfinished. But once the longing to know about every direction, every moment in time, everything — is finally put to rest by clearing away every last borrowed identity, then what remains is like a block of rock-salt: one taste all the way through, a solid mass of pure awareness, nothing inside it and nothing outside it. 'I am Brahman, the truth behind the truth' — at that point, the wanting-to-know stops completely, on every side, and awareness simply comes to rest in the Self, nowhere else. That's why the doubled 'not this, not this' exists — to negate everything, exhaustively. But surely, by a great —
Close to the Sanskrit
And this pair of negatives serves the purpose of vīpsā (repeated, distributive emphasis) and vyāpti (comprehensive pervasion): whatever, whatever is proposed as a possible referent, that, that is negated. And this being so, the doubt of 'Brahman being left unindicated' is removed. Otherwise, indeed, if the pair of negatives negated only the pair of things under discussion, then whatever Brahman is, other than the negated pair under discussion, would remain unindicated — and the doubt 'of what sort, then, indeed, is it?' would not be removed. And thus that designation would also be pointless, since it would fail to terminate the person's desire to know. And the statement 'I shall make Brahman known' would be of unfulfilled purpose. But when the desire to know regarding all directions, times, and so forth has been terminated by means of the removal of every condition-adjunct, then — like a lump of rock-salt, of a single essence, a solid mass of consciousness, without interior or exterior, 'I am Brahman, the truth of the truth' — the desire to know ceases entirely, and awareness comes to rest in the Self alone. Therefore, the pair of negatives 'not this, not this' serves the purpose of comprehensive negation. But surely, by a great —