Uddalaka is a familiar figure from both this Upanishad and the Brihad. In the previous section, for example, he is a brahmin approached by five householders who want to know about the atman vaishvanara (Olivelle translates as "The self of all men"). He doubts whether he will be able to answer them, however, and persuades them to join him in visiting the king Ashvapati. This is typical Uddalaka - he is usually the fall guy, the scholar who has a partial or merely conventional knowledge, and who is set up to contrast the true wisdom of someone else.
In Chapter VI however, Uddalaka is, unusually, himself the wise teacher for once. His son returns from 12 years' Vedic study proud and arrogant, and Uddalaka points out that there something he does not know - the knowledge of which allows one to understand everything.
Now, it might be imagined that this knowledge would be the knowledge of the Atman, and that is not untrue. But, as we are learning, the devil is always in the detail of the Upanishads. Almost all of the dialogues have the Atman as their subject, but they say very different things about it.
For example, we do not find Uddalaka speaking of the Atman as "neti, neti", as Yajnyawalkya does. Nor does he speak of it as the self of the cosmic being, as Ashvapati does. Nor do we find him speaking of it as the Brahman, as Shandilya does.
So the interest in this chapter is in what this knowledge is that Uddalaka speaks of, and how he does it, and why.
