Bhashya
आ नखाग्रेभ्यः-- नखाग्रमर्यादात्मनश्चैतन्यमुपलभ्यते ।तत्र कथमिव प्रविष्ट इत्याह-- यथा लोके, क्षुरधाने क्षुरो धीयते अस्मिन्निति क्षुरधानं तस्मिन् नापितोपस्कराधाने, क्षुरः अन्तस्थ उपलभ्यते-- अवहितः प्रवेशितः, स्यात्; यथा वा विश्वंभरः अग्निः-- विश्वस्य भरणात् विश्वंभरः कुलाये नीडे अग्निः काष्ठादौ, अवहितः स्यादित्यनुवर्तते ; तत्र हि स मध्यमान उपलभ्यते ।यथा च क्षुरः क्षुरधान एकदेशेऽवस्थितः, यथा चाग्निः काष्ठादौ सर्वतो व्याप्यावस्थितः, एवं सामान्यतो विशेषतश्च देहं संव्याप्यावस्थित आत्मा ; तत्र हि स प्राणनादिक्रियावान् दर्शनादिक्रियावांश्चोपलभ्यते ।तस्मात् तत्र एवं प्रविष्टं तम् आत्मानं प्राणनादिक्रियाविशिष्टम्, न पश्यन्ति नोपलभन्ते ।नन्वप्राप्तप्रतिषेधोऽयम्-- 'तं न पश्यन्ति' इति, दर्शनस्याप्रकृतत्वात् ; नैष दोषः ; सृष्ट्यादिवाक्यानामात्मैकत्वप्रतिपत्त्यर्थपरत्वात्प्रकृतमेव तस्य दर्शनम् ; 'रूपं रूपं प्रतिरूपो बभूव तदस्य रूपं प्रतिचक्षणाय' इति मन्त्रवर्णात् ।
In plain words
Consciousness of the self is experienced right down to the tips of the nails. How, then, has it entered the body? The text explains through familiar examples: just as a razor is safely tucked inside a barber's case, or just as fire lies latent within pieces of wood—revealed only when it is churned. Just as a razor occupies only a corner of its case while fire permeates every part of the fuel it inhabits, so the self pervades this entire body both generally and specifically, revealing itself through vital functions like breathing and seeing. Yet, because of this subtlety, people do not see or recognize the self thus present within them. If someone objects that the scriptural statement 'they do not see him' prohibits something that was never even under discussion, that objection fails. The passages on creation are ultimately meant to reveal the oneness of the self, making the perception of the self the central subject all along, as confirmed by the Vedic verse: 'He assumed every form as his own reflection, that he might be perceived.'
Close to the Sanskrit
Up to the nail-tips, consciousness is perceived of the self. Here, how as it were has it entered, he says: just as in the world, a razor is placed in a razor-container, namely in the barber's equipment case, the razor is found inside—inserted, it might be; or just as the all-sustaining fire—from sustaining the universe, all-sustaining—fire in the nest, in wood and the like, might be inserted, as it continues; for there it is perceived when churned. And just as a razor is situated in one part of a razor-container, and just as fire is situated pervading everywhere in wood and the like, so the self is situated having pervaded the body generally and specifically; for there indeed it is perceived possessing activities such as breathing and seeing. Therefore, they do not see or perceive that self thus entered and characterized by breathing and other activities. But is this not a prohibition of what is not attained—'him they do not see', since seeing was not under discussion? This is no fault, because creation and other sentences have the realization of the unity of the self as their purport, so his seeing is indeed contextually established; from the mantra text: 'Form by form he became its counterpart; that is his form for the sake of beholding.'