Many there are who do not even hear of Atman; though hearing of Him, many do not comprehend. Wonderful is the expounder and rare the hearer; rare indeed is the experiencer of Atman taught by an able preceptor.
Atman, when taught by an inferior person, is not easily comprehended, because It is diversely regarded by disputants. But when It is taught by him who has become one with Atman, there can remain no more doubt about It. Atman is subtler than the subtlest and not to be known through argument.
This Knowledge cannot be attained by reasoning. Atman becomes easy of comprehension, O dearest, when taught by another. You have attained this Knowledge now. You are, indeed, a man of true resolve. May we always have an inquirer like you!
Yajur Veda, Katha Upanishad, Part One, Chapter II, 11