Bhashya
तद् तत्र एतस्मिन्, यथा- प्रदर्शितेऽर्थे दृष्टान्तोऽयमुपा-दीयते- यथा लोके महामत्स्यः, महांश्चासौ मत्स्यश्च, ना-देयेन स्रोतसा अहार्य इत्यर्थः, स्रोतश्च विष्टम्भयति, स्व-च्छन्दचारी, उभे कूले नद्याः पूर्वं च अपरं च अनुक्रमेणसंचरति ; संचरन्नपि कूलद्वयं तन्मध्यवर्तिना उदकस्त्रोतोवे-गेन न परवशी क्रियते- एवमेव अयं पुरुषः एतौ उभौअन्तौ अनुसंचरति ; कौ तौ ? स्वप्नान्तं च बुद्धान्तं च ।दृष्टान्तप्रदर्शनफलं तु- मृत्युरूपः कार्यकारणसंघातः सहतत्प्रयोजकाभ्यां कामकर्मभ्याम् अनात्मधर्मः ; अयं च आत्माएतस्माद्विलक्षणः- इति विस्तरतो व्याख्यातम् ॥
In plain words
So, still on the same point, here is the illustration promised: think of a great fish in the world — great, and a fish — one that no current, however strong, can carry off. It holds its own against the current, swimming exactly where it pleases, ranging in turn along both banks of the river, the near one and the far one. And even while it ranges across both banks, the force of the water rushing between them never once masters it. This person moves in just the same way between two shores. Which two? The shore of dream, and the shore of waking. Here is the whole point of the picture: this complex of body and faculties, wearing the face of death, together with the desire and action that drive it, belongs to something other than the Self. And this Self is different from all of that — that is what has now been explained in full.
Close to the Sanskrit
There, in this connection, in the matter as already shown, this illustration is now adduced: as in the world a great fish — he is great, and he is a fish — is not to be carried off by a current that cannot be forded, that is the meaning; and it withstands the current, moving as it wishes, ranging in turn over both banks of the river, the former and the other; even while ranging over the pair of banks, by the force of the water-current lying between them it is not made subject to another — exactly so does this person move along these two ends. Which two? The dream-end and the waking-end. Now the point of presenting this illustration is: the aggregate of cause and effect that wears the form of death, together with the desire and action that impel it, is a property not of the Self; and this Self is distinct from that — thus has it been explained at length.