Bhashya
कथं पुनः इदानीम् इदं सर्वमात्मैवेति ग्रहीतुं शक्यते?चिन्मात्रानुगमात्सर्वत्र चित्स्वरूपतैवति गम्यते; तत्र दृ-ष्टान्त उच्यते— यत्स्वरूपव्यतिरेकेणाग्रहणं यस्य, तस्यतदात्मत्वमेव लोके दृष्टम् ; स यथा— स इति दृष्टान्तः ;लोके यथा दुन्दुभेः भेर्यादेः, हन्यमानम्य ताड्यमानस्यदण्डादिना, न, बाह्यान् शब्दान् बहिर्भूतान् शब्दविशेषान्दुन्दुभिशब्दसामान्यान्निष्कृष्टान् दुन्दुभिशब्दविशेषान्, नशक्नुयात् ग्रहणाय ग्रहीतुम् ; दुन्दुभेस्तु ग्रहणेन, दु-न्दुभिशब्दसामान्यविशेषत्वेन, दुन्दुभिशब्दा एते इति,शब्दविशेषा गृहीता भवन्ति, दुन्दुभिशब्दसामान्यव्य-तिरेकेणाभावात् तेषाम् ; दुन्दुभ्याघातस्य वा, दुन्दुभेरा-हननम् आघातः— दुन्दुभ्याघातविशिष्टस्य शब्दसामान्यस्यग्रहणेन तद्गता विशेषा गृहीता भवन्ति, न तु त एव नि-र्भिद्य ग्रहीतुं शक्यन्ते, विशेषरूपेणाभावात् तेषाम्—तथाप्रज्ञात्वव्यतिरेकेण स्वप्नजागरितयोः न कश्चिद्वस्तुविशेषो गृ-ह्यते ; तस्मात् प्रज्ञानव्यतिरेकेण अभावो युक्तस्तेषाम् ॥
In plain words
But how, exactly, can we grasp that all this really is the Self alone? Because consciousness alone runs through everything, we come to see that everywhere, at bottom, the nature of things is consciousness itself. Here's the illustration for that. Whenever the particulars of a thing can never be caught apart from its general nature, that general nature is simply what those particulars are — this is something we see everywhere in ordinary life. The word 'it' here is just a way of pointing to the example, nothing more. Think of a drum, or a kettle-drum, being struck with a stick. You can never grab hold of its many individual sounds as separate things in their own right, apart from the one general thing, 'drum-sound.' But grasp the drum itself, and you've grasped them all at once — because, being nothing but instances of that one drum-sound, they have no existence apart from it. Or grasp the beating of the drum, and the particular sounds carried within that beating are grasped along with it — though you could never split them apart and hold each one on its own, because as separate things they simply aren't there. It's the same with dream and waking. Apart from consciousness itself, no particular object is ever actually found in either state. So it follows: nothing exists apart from consciousness.
Close to the Sanskrit
How again, now, can it be grasped that all this is the Self alone? Because of the accompaniment of consciousness alone everywhere, it is understood that everywhere the very nature is consciousness alone. In that connection an illustration is stated: of whatever thing the non-apprehension of whose [particulars] apart from its own general nature occurs, of that general nature, being identical with it is precisely what is observed in the world. 'Sa' [in 'sa yathā'] — the word 'sa' is merely the marker of an illustration. As in the world, of a drum or kettle-drum being struck, being beaten with a stick or the like, one is not able to grasp for apprehension the external sounds — sounds that have become external, particular sounds singled out as distinct from the universal 'drum-sound,' namely the particular instances of drum-sound; but by the grasping of the drum, inasmuch as they are particulars of the universal 'drum-sound' — [thinking] 'these are drum-sounds' — the particular sounds come to be grasped, because of their non-existence apart from the universal 'drum-sound'; or, by the grasping of the beating of the drum — the striking of the drum is called the beating — by the grasping of the universal 'sound' as qualified by the beating of the drum, the particulars belonging to it come to be grasped; but those very particulars cannot be grasped by breaking them apart, because of their non-existence in the form of separate particulars. Likewise, apart from the state of being consciousness, in dream and in waking, no particular object whatsoever is perceived; therefore the non-existence of those [particulars] apart from consciousness is rightly to be concluded.