Verse 1.1.20

येयं प्रेते विचिकित्सा मनुष्येऽस्तीत्येके नायमस्तीति चैके ।
एतद्विद्यामनुशिष्टस्त्वयाऽहं वराणामेष वरस्तृतीयः ॥ २० ॥

yeyaṃ prete vicikitsā manuṣye'stītyeke nāyamastīti caike |
etadvidyāmanuśiṣṭastvayā'haṃ varāṇāmeṣa varastṛtīyaḥ || 20 ||

20. (Nachiketas said) This well-known doubt as to what becomes of a man after death,—some say he is and some, he is not,—I shall know being taught by thee. This boon is the third of the boons.

Shankara’s Commentary:

Com.—Thus much, which has been indicated by the two boons, is alone to be understood from the preceding Mantras and Brahmanas, which are mandatory or prohibitory in their import but not the knowledge of the true nature of the entity of the atman. Therefore, for the dispelling of natural ignorance (Avidya), which deals with matters the subject of positive and prohibitory injunctions, which consists in superposing on the atman, activity, agency and enjoyment, and which is the seed of samsara, it is necessary to explain the knowledge of the identity of the Brahman and the atman, which is contrary to that previously explained which is free from the fault of superposition of activity; agency, and enjoyment on the atman, and whose result is the attainment of absolute emancipation; with this end, the subsequent portion of this work is commenced. The anecdote explains how without this well-known knowledge of the atman, the subject of the third boon, all that is desirable is not achieved even by the obtaining of the second boon. Because, it is only those, who are disgusted with the result previously named in the nature of means and ends, transitory, and produced by karma, that are entitled to acquire the knowledge of the atman; therefore, to denounce Karma, it is sought to tempt Nachiketas away from his object, by promise of sons and the rest. Nachiketas being asked by Death to name his third boon, said: ‘This doubt regarding man when dead,—some say that there is an atman distinct from body, senses, mind and intellect and entering into another body; some say that there is no such atman; and this doubt cannot be resolved by us, either by direct perception or logical inference; and because the attainment of the highest consummation depends upon a clear knowledge of this. I would acquire this knowledge, being instructed by thee. This, the third, i.e., the last of the boons.