The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
19. (The Self to be seen, to be heard, &c. is the highest Self) on account of the connected meaning of the sentences.
We read in the Brihadâranyaka, in the Maitreyî-brâhmana the following passage, 'Verily, a husband is not dear that you may love the husband, &c. &c.; verily, everything is not dear that you may love everything; but that you may love the Self therefore everything is dear. Verily, the Self is to be seen, to be heard, to be perceived, to be marked, O Maitreyî! When the Self has been seen, heard, perceived, and known, then all this is known' (Bri. Up. IV, 5, 6).--Here the doubt arises whether that which is represented as the object to be seen, to be heard, and so on, is the cognitional Self (the individual soul) or the highest Self.--But whence the doubt?--Because, we reply, the Self is, on the one hand, by the mention of dear things such as husband and so on, indicated as the enjoyer whence it appears that the passage refers to the individual soul; and because, on the other hand, the declaration that through the knowledge of the Self everything becomes known points to the highest Self.
The pûrvapakshin maintains that the passage refers to the individual soul, on account of the strength of the initial statement. The text declares at the outset that all the objects of enjoyment found in this world, such as husband, wife, riches, and so on, are dear on account of the Self, and thereby gives us to understand that the enjoying (i.e. the
individual) Self is meant; if thereupon it refers to the Self as the object of sight and so on, what other Self should it mean than the same individual Self?--A subsequent passage also (viz. 'Thus does this great Being, endless, unlimited. consisting of nothing but knowledge, rise from out of these elements, and vanish again after them. When he has departed there is no more knowledge'), which describes how the great Being under discussion rises, as the Self of knowledge, from the elements, shows that the object of sight is no other than the cognitional Self, i.e. the individual soul. The concluding clause finally, 'How, O beloved, should he know the knower?' shows, by means of the term 'knower,' which denotes an agent, that the individual soul is meant. The declaration that through the cognition of the Self everything becomes known must therefore not be interpreted in the literal sense, but must be taken to mean that the world of objects of enjoyment is known through its relation to the enjoying soul.
To this we make the following reply.--The passage makes a statement about the highest Self, on account of the connected meaning of the entire section. If we consider the different passages in their mutual connexion, we find that they all refer to the highest Self. After Maitreyî has heard from Yâgñavalkya that there is no hope of immortality by wealth, she expresses her desire of immortality in the words, 'What should I do with that by which I do not become immortal? What my Lord knoweth tell that to me; and thereupon Yâgñavalkya expounds to her the knowledge of the Self. Now Scripture as well as Smriti declares that immortality is not to be reached but through the knowledge of the highest Self.--The statement further that through the knowledge of the Self everything becomes known can be taken in its direct literal sense only if by the Self we understand the highest cause. And to take it in a non-literal sense (as the pûrvapakshin proposes) is inadmissible, on account of the explanation given of that statement in a subsequent passage, viz. 'Whosoever looks for the Brahman class elsewhere than in the Self, is abandoned by the Brahman class.' Here it is said that whoever
erroneously views this world with its Brahmans and so on, as having an independent existence apart from the Self, is abandoned by that very world of which he has taken an erroneous view; whereby the view that there exists any difference is refuted. And the immediately subsequent clause, 'This everything is the Self,' gives us to understand that the entire aggregate of existing things is non-different from the Self; a doctrine further confirmed by the similes of the drum and so on.--By explaining further that the Self about which he had been speaking is the cause of the universe of names, forms, and works ('There has been breathed forth from this great Being what we have as Rig-veda,' &c.) Yâgñavalkya again shows that it is the highest Self.--To the same conclusion he leads us by declaring, in the paragraph which treats of the natural centres of things, that the Self is the centre of the whole world with the objects, the senses and the mind, that it has neither inside nor outside, that it is altogether a mass of knowledge.--From all this it follows that what the text represents as the object of sight and so on is the highest Self.
We now turn to the remark made by the pûrvapakshin that the passage teaches the individual soul to be the object of sight, because it is, in the early part of the chapter denoted as something dear.