Sri Aurobindo translates Katha Upanishad, First Cycle: Second Chapter, Verses 7-9: “He that is not easy even to be heard of by many, and even of those that have heard, they are many who have not known Him, — a miracle is the man that can speak of Him wisely or is skilful to win Him, and when one is found, a miracle is the listener who can know God even when taught of Him by the knower. An inferior man cannot tell you of Him; for thus told thou canst not truly know Him, since He is thought of in many aspects. Yet unless told of Him by another thou canst not find thy way there to Him; for He is subtler than subtlety and that which logic cannot reach. This wisdom is not to be had by reasoning, O beloved Nachiketas; only when told thee by another it brings real knowledge, — the wisdom which thou hast gotten. Truly thou art steadfast in the Truth! Even such a questioner as thou art may I meet with always.”
The Upanishads generally hold that there is a higher knowledge and a lower knowledge. The higher knowledge is the knowledge of Brahman, the Transcendent, the Universal and the Immanent Divine. The lower knowledge is the knowledge of the forms, forces and actions in the outer world of Matter, Life and Mind. They categorize those of superior intelligence as the few who seek the higher knowledge. Inferior intelligence is fixated upon the lower knowledge. In the outer world, the functions of logic, reasoning and analytical intelligence are supreme and people debate truth using these tools and wind up in verbal and intellectual dead-ends without true knowledge. In the spiritual seeking, reliance upon logic and verbal acrobatics is not sufficient. Only those who have the experience of the Brahman can communicate it to another.
The tradition of the Guru in India is founded on the idea that an individual, on his own, would find it almost impossible to break out of the limits of the framework of the life of society. An illumined teacher who has the experience of the Brahman has the ability to communicate this truth to the disciple who is ready to receive it, not necessarily through the use of words, but through the force of the experience which informs his words and deeds. When one is in the presence of an individual who has the experience of the truth of the Spirit, there is an activating force that a receptive seeker can receive and which attunes him to the spiritual truth of existence.
For the vast majority of people who are focused solely on the outer life of desire and ego, the development of the lower knowledge, the knowledge of the world and its ways, is their constant preoccupation. Few are those who seek to go beyond this to comprehend “that by knowing which all is known” in the quest for spiritual understanding and realisation.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129