The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
12. (The Self) consisting of bliss (is the highest Self) on account of the repetition (of the word 'bliss,' as denoting the highest Self).
The Taittirîya-upanishad (II, 1-5), after having enumerated the Self consisting of food, the Self consisting of the vital airs, the Self consisting of mind, and the Self consisting of understanding, says, 'Different from this which consists of understanding is the other inner Self which consists of bliss.' Here the doubt arises whether the phrase, 'that which consists of bliss,' denotes the highest Brahman of which it had been said previously, that 'It is true Being, Knowledge, without end,' or something different from Brahman, just as the
[paragraph continues] Self consisting of food, &c., is different from it.--The pûrvapakshin maintains that the Self consisting of bliss is a secondary (not the principal) Self, and something different from Brahman; as it forms a link in a series of Selfs, beginning with the Self consisting of food, which all are not the principal Self. To the objection that even thus the Self consisting of bliss may be considered as the primary Self, since it is stated to be the innermost of all, he replies that this cannot be admitted, because the Self of bliss is declared to have joy and so on for its limbs, and because it is said to be embodied. If it were identical with the primary Self, joy and the like would not touch it; but the text expressly says 'Joy is its head;' and about its being embodied we read, 'Of that former one this one is the embodied Self' (Taitt. Up. II, 6), i.e. of that former Self of Understanding this Self of bliss is the embodied Self. And of what is embodied, the contact with joy and pain cannot be prevented. Therefore the Self which consists of bliss is nothing but the transmigrating Soul.
To this reasoning we make the following reply:--By the Self consisting of bliss we have to understand the highest Self, 'on account of repetition.' For the word 'bliss' is repeatedly applied to the highest Self. So Taitt. Up. II, 7, where, after the clause 'That is flavour'--which refers back to the Self consisting of bliss, and declares it to be of the nature of flavour--we read, 'For only after having perceived flavour can any one perceive delight. Who could breathe, who could breathe forth if that Bliss existed not in the ether (of the heart)? For he alone causes blessedness;' and again, II, 8, 'Now this is an examination of Bliss;' 'He reaches that Self consisting of Bliss;' and again, II, 9, 'He who knows the Bliss of Brahman fears nothing;' and in addition, 'He understood that Bliss is Brahman' (III, 6). And in another scriptural passage also (Bri. Up. III, 9, 28), 'Knowledge and bliss is Brahman,' we see the word 'bliss' applied just to Brahman. As, therefore, the word 'bliss' is repeatedly used with reference to Brahman, we conclude that the Self consisting of bliss is Brahman also. The objection that the Self consisting of bliss can only denote
the secondary Self (the Samsârin), because it forms a link in a series of secondary Selfs, beginning with the one consisting of food, is of no force, for the reason that the Self consisting of bliss is the innermost of all. The Sâstra, wishing to convey information about the primary Self, adapts itself to common notions, in so far as it at first refers to the body consisting of food, which, although not the Self, is by very obtuse people identified with it; it then proceeds from the body to another Self, which has the same shape with the preceding one, just as the statue possesses the form of the mould into which the molten brass had been poured; then, again, to another one, always at first representing the Non-Self as the Self, for the purpose of easier comprehension; and it finally teaches that the innermost Self 1, which consists of bliss, is the real Self. Just as when a man, desirous of pointing out the star Arundhatî to another man, at first points to several stars which are not Arundhatî as being Arundhatî, while only the star pointed out in the end is the real Arundhatî; so here also the Self consisting of bliss is the real Self on account of its being the innermost (i.e. the last). Nor can any weight be allowed to the objection that the attribution of joy and so on, as head, &c., cannot possibly refer to the real Self; for this attribution is due to the immediately preceding limiting condition (viz. the Self consisting of understanding, the so-called vigñânakosa), and does not really belong to the real Self. The possession of a bodily nature also is ascribed to the Self of bliss, only because it is represented as a link in the chain of bodies which begins with the Self consisting of food, and is not ascribed to it in the same direct sense in which it is predicated of the transmigrating Self. Hence the Self consisting of bliss is the highest Brahman.
66:1 After which no other Self is mentioned.