The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
6. And there is question and explanation relative to three things only (not to the pradhâna).
To the same conclusion we are led by the consideration of the circumstance that the Kathavallî-upanishad brings forward, as subjects of discussion, only three things, viz. the fire sacrifice, the individual soul, and the highest Self. These three things only Yama explains, bestowing thereby the boons he had granted, and to them only the questions of Nakiketas refer. Nothing else is mentioned or enquired
about. The question relative to the fire sacrifice is contained in the passage (Ka. Up. I, 1, 13), 'Thou knowest, O Death, the fire sacrifice which leads us to Heaven; tell it to me, who am full of faith.' The question as to the individual soul is contained in I, 1, 20, 'There is that doubt when a man is dead, some saying, he is; others, he is not. This I should like to know, taught by thee; this is the third of my boons.' And the question about the highest Self is asked in the passage (I, 2, 14), 'That which thou seest as neither this nor that, as neither effect nor cause, as neither past nor future, tell me that.'--The corresponding answers are given in I, 1, 15, 'Yama then told him that fire sacrifice, the beginning of all the worlds, and what bricks are required for the altar, and how many;' in the passage met with considerably later on (II, 5, 6; 7), 'Well then, O Gautama, I shall tell thee this mystery, the old Brahman and what happens to the Self after reaching death. Some enter the womb in order to have a body as organic beings, others go into inorganic matter according to their work and according to their knowledge;' and in the passage (I, 2, 18), 'The knowing Self is not born nor does it die,' &c.; which latter passage dilates at length on the highest Self. But there is no question relative to the pradhâna, and hence no opportunity for any remarks on it.
Here the Sânkhya advances a new objection. Is, he asks, the question relative to the Self which is asked in the passage, 'There is that doubt when a man is dead,' &c., again resumed in the passage, 'That which thou seest as neither this nor that,' &c, or does the latter passage raise a distinct new question? If the former, the two questions about the Self coalesce into one, and there are therefore altogether two questions only, one relative to the fire sacrifice, the other relative to the Self. In that case the Sûtra has no right to speak of questions and explanations relating to three subjects.--If the latter, you do not consider it a mistake to assume a question in excess of the number of boons granted, and can therefore not object to us if we assume an explanation about the pradhâna in excess of the number of questions asked.
To this we make the following reply.--We by no means assume a question in excess of the number of boons granted, being prevented from doing so by the influence of the opening part of that syntactical whole which constitutes the Kathavallî-upanishad. The Upanishad starts with the topic of the boons granted by Yama, and all the following part of the Upanishad--which is thrown into the form of a colloquy of Yama and Nakiketas--carries on that topic up to the very end. Yama grants to Nakiketas, who had been sent by his father, three boons. For his first boon Nakiketas chooses kindness on the part of his father towards him, for his second boon the knowledge of the fire sacrifice, for his third boon the knowledge of the Self. That the knowledge of the Self is the third boon appears from the indication contained in the passage (I, 1, 20), 'There is that doubt--; this is the third of my boons.'--If we therefore supposed that the passage, 'That which thou seest as neither this nor that,' &c., raises a new question, we should thereby assume a question in excess of the number of boons granted, and thus destroy the connexion of the entire Upanishad.--But-the Sânkhya will perhaps interpose--it must needs be admitted that the passage last quoted does raise a new question, because the subject enquired about is a new one. For the former question refers to the individual soul, as we conclude from the doubt expressed in the words, 'There is that doubt when a man is dead--some saying, he is; others, he is not.' Now this individual soul, as having definite attributes, &c., cannot constitute the object of a question expressed in such terms as, 'This which thou seest as neither this nor that,' &c.; the highest Self, on the other hand, may be enquired about in such terms, since it is above all attributes. The appearance of the two questions is, moreover, seen to differ; for the former question refers to existence and non-existence, while the latter is concerned with an entity raised above all definite attributes, &c. Hence we conclude that the latter question, in which the former one cannot be recognised, is a separate question, and does not merely resume the subject of the former one.--All this argumentation is not valid, we reply, since we maintain the unity of the highest Self and the
individual Self. If the individual Self were different from the highest Self, we should have to declare that the two questions are separate independent questions, but the two are not really different, as we know from other scriptural passages, such as 'Thou art that.' And in the Upanishad under discussion also the answer to the question, 'That which thou seest as neither this nor that,' viz. the passage, 'The knowing Self is not born, it dies not'--which answer is given in the form of a denial of the birth and death of the Self-clearly shows that the embodied Self and the highest Self are non-different. For there is room for a denial of something only when that something is possible, and the possibility of birth and death exists in the embodied Self only, since it is connected with the body, but not in the highest Self.--There is, moreover, another passage conveying the same meaning, viz. II, 4, 4, 'The wise when he knows that that by which he perceives all objects in sleep or in waking, is the great omnipresent Self, grieves no more.' This passage makes the cessation of all grief dependent on the knowledge of the individual Self, in so far as it possesses the qualities of greatness and omnipresence, and thereby declares that the individual Self is not different from the highest Self. For that the cessation of all sorrow is consequent on the knowledge of the highest Self, is a recognised Vedânta tenet.--There is another passage also warning men not to look on the individual Self and the highest Self as different entities, viz. II, 4, 10, 'What is here the same is there; and what is there the same is here. He who sees any difference here goes from death to death.'--The following circumstance, too, is worthy of consideration. When Nakiketas has asked the question relating to the existence or non-existence of the soul after death, Yama tries to induce him to choose another boon, tempting him with the offer of various objects of desire. But Nakiketas remains firm. Thereupon Death, dwelling on the distinction of the Good and the Pleasant, and the distinction of wisdom and ignorance, praises Nakiketas, 'I believe Nakiketas to be one who desires knowledge, for even many pleasures did not tear thee away' (I, 2, 4); and later on praises the question
asked by Nakiketas, 'The wise who, by means of meditation on his Self, recognises the Ancient who is difficult to be seen, who has entered into the dark, who is hidden in the cave, who dwells in the abyss, as God, he indeed leaves joy and sorrow far behind' (I, 2, 12). Now all this means to intimate that the individual Self and the highest Self are non-different. For if Nakiketas set aside the question, by asking which he had earned for himself the praise of Yama, and after having received that praise asked a new question, all that praise would have been bestowed on him unduly. Hence it follows that the question implied in I, 2, 14, 'That which thou seest as neither this nor that,' merely resumes the topic to which the question in I, 1, 20 had referred.--Nor is there any basis to the objection that the two questions differ in form. The second question, in reality, is concerned with the same distinction as the first. The first enquires about the existence of the soul apart from the body, &c.; the second refers to the circumstance of that soul not being subject to samsâra. For as long as Nescience remains, so long the soul is affected with definite attributes, &c.; but as soon as Nescience comes to an end, the soul is one with the highest Self, as is taught by such scriptural texts as 'Thou art that.' But whether Nescience be active or inactive, no difference is made thereby in the thing itself (viz. the soul). A man may, in the dark, mistake a piece of rope lying on the ground for a snake, and run away from it, frightened and trembling; thereon another man may tell him, 'Do not be afraid, it is only a rope, not a snake;' and he may then dismiss the fear caused by the imagined snake, and stop running. But all the while the presence and subsequent absence of his erroneous notion, as to the rope being a snake, make no difference whatever in the rope itself. Exactly analogous is the case of the individual soul which is in reality one with the highest soul, although Nescience makes it appear different. Hence the reply contained in the passage, 'It is not born, it dies not,' is also to be considered as furnishing an answer to the question asked in I, 1, 20.--The Sutra is to be understood with reference to the distinction of the individual Self and the highest Self which
results from Nescience. Although the question relating to the Self is in reality one only, yet its former part (I, 1, 20) is seen specially to refer to the individual Self, since there a doubt is set forth as to the existence of the soul when, at the time of death, it frees itself from the body, and since the specific marks of the samsâra-state, such as activity, &c. are not denied; while the latter part of the question (I, 2, 14), where the state of being beyond all attributes is spoken of, clearly refers to the highest Self.--For these reasons the Sûtra is right in assuming three topics of question and explanation, viz. the fire sacrifice, the individual soul, and the highest Self. Those, on the other hand, who assume that the pradhâna constitutes a fourth subject discussed in the Upanishad, can point neither to a boon connected with it, nor to a question, nor to an answer. Hence the pradhâna hypothesis is clearly inferior to our own.