In the Kena Upaniṣad, the teacher attempts to impart knowledge of Brahman. The eyes, speech, and mind cannot reach It. An object is ordinarily cognised through the mind and senses. Since Brahman is not an object of perception, the teacher says: We do not know Brahman as an object that can be described as such and such, nor are we aware of any process by which It can be taught as an object.

An object perceived by the senses can be taught by classifying it on the basis of its class, qualities, and actions. Brahman, however, does not fall within these categories. Therefore, the methods ordinarily employed to teach about objects are unsuitable for imparting knowledge of Brahman.

A different approach is required to impart this knowledge and bring the truth within the seeker’s comprehension. This is provided by the traditional teaching handed down by the ancient teachers. They have described Brahman through apparently paradoxical statements:

Brahman is different from the known and also different from the unknown. It is neither known nor unknown.

Shankarcharya resolves this apparent paradox.

Whatever is known as an object is limited, mortal, and subject to misery; hence, it is to be rejected as the ultimate reality. Therefore, when Brahman is said to be different from the known, it amounts to saying that It is not something to be rejected. Similarly, when Brahman is said to be different from the unknown, it means that It is not something to be attained as an object.                                                                                                

Since Brahman is neither an object to be acquired nor one to be rejected, the disciple’s desire to know Brahman objectively comes to an end. Instead, he understands the teacher’s instruction that nothing other than one’s own Self can be different from both the known and the unknown.
In this way, the teacher successfully imparts the knowledge that Brahman is the very Self of the seeker, which is pure consciousness.

Shankaracharya affirms the effectiveness of the above method.

 Brahman is to be known only through the traditional instruction imparted by preceptors. It cannot be known through mere argumentation, study or exposition, intelligence, extensive learning, austerity, sacrifices, or similar means.

Note: Main source: Eight Upaniṣhads with the Commentary of Shankaracharya translated by Swami Gambhirananda, Advaita Ashrama.