Ātma can’t be known by logic

Why ātmā need to be taught in the manner mentioned? aṇīyān hyatarkyamaṇupramāṇāt - because ātmā is subtler than the subtlest and not available for reasoning. In śāstra, the subtlety of a given object is proportional to the number of properties it has. Five elements are forming all the manifested objects in the world, namely space, air, fire, water and earth. Out of them, the earth has five properties - sound, touch, form, taste and smell, thus it is the grossest. Water is the next grossest because it has all the properties except for smell. Fire, air, and space are next in order of subtlety. Space has only one properly, which is sound, therefore it is the subtlest among all. Ātmā is even subtler than space because as the very un-differentiated existence of these elements, there is no property/attribute on the part of ātmā.

This subtlest ātma can’t be known by logic or reasoning, then with what means can I know ātma?

naiṣā tarkeṇa matirāpaneyā proktānyenaiva sujñānāya preṣṭha |

yām tvamāpaḥ satyadhṛtirbatāsi tvādṛṅ no bhūyānnaciketaḥ praṣṭā ||1.2.9||

This knowledge cannot be attained by reasoning, oh dear one! When taught by a competent teacher, it leads to clear and firm knowledge. You have attained that knowledge. You are indeed a resolute one. O naciketa! May we have a seeker like you.

Yamaraja confirms once more that naiṣā tarkeṇa matirāpaneyā - ātmā can’t be gained by mere logic which is speculation of one’s intellect. Because ātmā is not in the scope of human intellect, and only śāstra is the means to know ātmā. Therefore proktānyenaiva sujñānāya - it can be understood very well only through a competent teacher, who knows how to handle the meaning of sentences of śāstra thus they become clear and firm understanding for students.

And yamaraja said yām tvamāpaḥ - this self-knowledge indeed has been attained by you (naciketa) through the granting of the third boon by yamaraja. Even though knowledge is yet to be taught, yamaraja is certain that naciketa will attain this knowledge because satyadhṛtirbatāsi - naciketa has determination only for the truth. This indicates the mumuksutvam - intense desire as the qualification for moksa on the part of the student. Yamaraja is also confident that he is a competent teacher. For these two reasons, successful teaching will take place. At last, yamaraja expressed his enthusiasm to unfold this knowledge, tvādṛṅ no bhūyānnaciketaḥ praṣṭā - may I have the questioner/disciple like you, who created the sense of the importance of qualifications for self-knowledge.