Bhashya
शरीरे, इन्द्रियसंपर्कात् ।एवं पारम्पर्येण कृत्स्नं कार्यकार-णसंघातम् आत्मा चैतन्यस्वरूपज्योतिषा अवभासयति ।तेनहि सर्वस्य लोकस्य कार्यकारणसंघाते तद्वृत्तिषु च अनि-यतात्माभिमानबुद्धिः यथाविवेकं जायते ।तथा च भग-वतोक्तं गीतासु— 'यथा प्रकाशयत्येकः कृत्स्नं लोकमिमंरविः ।क्षेत्रं क्षेत्री तथा कृत्स्नं प्रकाशयति भारत''यदादित्यगतं तेजः——' इत्यादि च ।'नित्योऽनित्यानांचेतनश्चेतनानाम्' इति च काठके, 'तमेव भान्तमनुभातिसर्वं तस्य भासा सर्वमिदं विभाति' इति च ।'येन सूर्य-स्तपति तेजसेद्धः' इति च मन्त्रवर्णः ।तेनायं हृद्यन्तर्ज्यो-तिः ।पुरुषः— आकाशवत्सर्वगतत्वात् पूर्ण इति पुरुषः ;निरतिशयं च अस्य स्वयंज्योतिष्ट्वम्, सर्वावभासकत्वात् स्व-यमन्यानवभास्यत्वाच्च ; स एष पुरुषः स्वयमेव ज्योतिःस्व-भावः, यं त्वं पृच्छसि- कतम आत्मेति ॥
In plain words
In the body, through contact with the senses — this is how it works. Step by step, the Self lights up this whole structure of causes and effects with a light that isn't borrowed from anywhere: it is consciousness itself, shining as its own nature. And because of that light, everyone in this world keeps forming a shifting notion of self, pinning it now on the body, now on its states, always in proportion to how little they've learned to tell the two apart. The Lord says the same thing in the Gītā: 'As one sun lights up this whole world, so the knower of the field lights up the whole field, Bhārata' — and, 'The radiance living in the sun...' The Kaṭha Upaniṣad says it too: 'The eternal among the fleeting, the conscious among the conscious.' And elsewhere: 'Only by his shining does anything else shine at all; his light is what lights up this whole world.' Even the old mantras say it: 'The sun burns because it has already been set ablaze by that splendour.' So this is the light within the heart. And 'Person' — because like space it reaches everywhere, it is full, and that fullness is why it's called Person. Its self-luminous nature has no rival: it lights up everything else, and nothing lights it up in return. That very Person is light through and through — and that is exactly what you're asking about when you ask, 'Which one is the Self?'
Close to the Sanskrit
In the body, because of contact with the senses — thus, by succession, the Self illumines the entire aggregate of effect and cause with the light that is its own nature as consciousness. By that indeed, for the whole world, in the aggregate of effect and cause and in its states, an unfixed notion of self-identification arises according to one's discrimination. And so it has been said by the Lord in the Gītā: 'As the one sun illumines this entire world, so the knower of the field illumines the entire field, O Bhārata,' and 'The splendour that is in the sun...' and so on. 'The eternal among the eternal, the conscious among the conscious' — thus also in the Kāṭhaka, and 'Him alone shining, everything shines after; by his light all this shines forth' — thus also. 'By whom the sun burns, kindled by that splendour' — thus also a Vedic mantra-passage. Therefore this one is 'the light within the heart.' Person — because, like space, it is all-pervading, it is full; hence 'Person.' And unsurpassed is its state of being self-luminous, because it illumines everything itself and because it is not illumined by anything else. That very Person is itself of the nature of light — the very one you ask about: 'Which is the Self?'